Another itinerary for your critique!

A few thoughts:

If you’re into Harry Potter at all, then apparently the Warner Brothers site is amazing but it needs booking weeks/months ahead. Very easy to get to from central London (fast train from Euston to Watford Junction).

Tower of London is worth at least a full morning and maybe more. You mean Tower Bridge not London Bridge. You will short-change both the Tower and Greenwich if you try to fit them in the same day in my opinion.

Do the London Eye when the weather is clear. It would also fit with a Thames Path day - the walk along the South Bank between the Eye and Tower Bridge is great, and passes a lot of interesting stuff: Tate Modern, the Globe, views of St Paul’s across the river.

The HOHO bus isn’t worth the time and money as it’s punishingly slow. However if you want to ride a double decker bus then the good news is that you can just ride upstairs on a normal London bus! Somewhere there’s a list of the best routes for tourists - I’ll try to find it. Sitting in the front upstairs seats of a London double decker bus for (can’t remember the current price but it’s about £3) is something that still thrills me and I’ve lived in London and have been visiting for years.

EDIT TO ADD BUS LINK: https://tfl.gov.uk/travel-information/visiting-london/experience-london/bus-leisure-routes

Oh, thank you this is FABULOUS...just what I pictured for a response!

"Piccadilly Circus & Fortnum & Mason (going back for tea later"

Piccadilly Circus is in my opinion completely missable, but I suspect you're meaning Piccadilly (the street) and surrounds (ie Regent St, Bond St etc. Given you've got Harrods and Fortnums on your list, how about adding Liberty? It's an amazing store, and worth it just to see the building alone.

Elsewhere k your itinerary, I can endorse Cinnamon Club - it's great.

The bus fare is £1.75 (as many buses as you like within 62 minutes of first tap in) capped at £5.25 a day.

Tube and train fares in London go up on Sunday, but bus fares are frozen.

The Thames Path goes all the way from way east of Greenwich to Thames Head, far beyond Oxford, takes days to walk. So you can choose any section of that. And through Windsor. You could walk from central London to Windsor on the path with a day or two to spare.

For instance pick the path up at Greenwich heading into London and then hop on a bus or the Uber boat/Thames Clipper at any intermediate pier when you run out of time or energy. The Uber boat runs all the way to Putney with lots of piers, so you can do lots of little bits of the Path in the morning or evening. I've walked the whole thing, but more recently have been Pier Hopping Putney to Tower [of London] as early as 7am and as late as 10pm. Almost as much fun (maybe more) as the buses.

There is also the Regents Canal which winds round north London from Brentford to Limehouse. And at Limehouse that picks up the Lee and Stort Navigation- 40 miles or so to Hertford and Bishop's Stortford

This is a very comprehensive list. Two suggestions- 1) spend a whole day in Greenwich- there is so much there- between the museums, market, Queen's House, the park, etc- it's a wonderful day out; 2) as mentioned above, the Thames Path goes all the way into (the edge of) the Cotswolds, so there are lots of sections you could do- if you are planning to visit Windsor anyway you could combine that with some of the Thames Path- you could start in Staines, walk thru where Magna Carta was signed and finish in Windsor and visit the castle. Or walk from Kingston to Hampton Court and visit Hampton Court. Good luck with the planning!

The Sherlock Holmes Pub is at Northumberland Street between Embankment and Charing Cross Stations- https://www.greeneking.co.uk/pubs/greater-london/sherlock-holmes

You could dine one evening at Simpsons in the Strand- his favourite restaurant.

Just a thought that Sunday might not be the best day for Leadenhall Market and the area around Liverpool Street. The City of London is very much the business district - a bit like Wall Street in NYC. I used to work in the City and once had occasion to be there on a Sunday. Many of my usual lunch haunts were closed and the place was basically 'dead'. I know Leadenhall Market is open so you can walk though, but again some of the shops might be closed. It might be worth shifting your plans a little so you can go there on a working day.

I'd also second the advice about not really needing much time for Picadilly Circus. I suppose it's a bit like Time Square - but I've always found it a bit trashy. F&M on the other hand - wonderful!

Otherwise looks like a fun trip.

For some reason, the Tfl list of buses didn’t include the number 26 which goes past a number of major tourist sites

https://tfl.gov.uk/bus/route/26/

(Note- Westminster Abbey and Westminster Cathedral are different places. The latter is a Roman Catholic cathedral finished in the early 1900s)

Assume you land at Heathrow. The advice is usually to book a car service. Not take a - very expensive - cab. The usual car services recommended here are:

www.blackberrycars.com
www.simplyairports.co.uk
www.justairports.com

(Have no personal experience, usually use public transport.)

You seem to be missing Hampton Court Palace. This is easy to get to, train 36 mins from Waterloo to station opposite the Palace. There’s a lot to see and it’ll take the best part of a full day. Forced to choose, I’d prefer it over Windsor or Greenwich.

It has very attractive gardens and grounds, so if possible go on a dry day and sun makes it even better.

The Houses of Parliament are extremely close to Westminster Abbey, You might want to see those two back to back, but if you're staying in the Westminster area, it may not really matter.

I am not a fan at all of birds'-eye views; I much prefer seeing things down at sidewalk level rather than looking down on rooftops. Therefore, my perspective here may be very skewed, but you seem to be spending a lot of time on this relatively short trip going to places that provide aerial views of the city--three times on Day 5 alone.

London Walks has a slate of many dozens of walking tours, with ten or more offered almost every day. I encourage you to take a look at what they're doing on the days you're in London to see what might interest you. I know there's at least one Sherlock Holmes-related tour and there may be something Harry Potterish. Several tours include the word "Royal" in their titles. As it happens, I haven't taken any of those tours, but I've probably a dozen, if not more, of their walks over the years. All were very good, and the price is right at 20 GBP per person. The guides are licensed; they don't make up tall tales just to entertain you unless they're sharing something akin to a legend, in which case they'll identify it as such. The tours usually get you into back streets where few tourists go. https://www.walks.com/

People speak very highly of the verger tour at Westminster Abbey; you might like to research that and see whether it might appeal to you. I haven't even set foot in the abbey yet, but I plan to do that, and take the tour, later this year.

I think the Changing of the Guard is usually not recommended these days. You have to show up really, really early to try to get a standing spot where you can actually see something. It has never sounded worth the time to me in a city where you could spend months without running out of things to do.

I gather from people who've visited Kensington Palace that it's a bit underwhelming. I've only walked through the gardens, which were fine but to my mind not special by UK standards. If you're in the neighborhood with some spare time, go for it, but neither the palace nor the gardens would be on my list of top sightseeing targets.

I try to focus on healthy food when I travel and often make some of my meals a salad box from Pret a Manger. I've heard good things about Leon as well but haven't found a convenient opportunity to try it. There are many Prets all over London but not a lot of Leons.

Visiting a functioning cathedral on a Sunday is not a great idea unless you want to go to a service - you can’t look round it as well if you do that. And St Paul’s fits very nicely with the Tower of London for one full day.
You also need a full day to get the most out of Greenwich. Do that on the Sunday and come back on the DLR when you can pick up the Sky Garden etc.
If you want to do the Harry Potter Studio tour book it NOW. It will take up most of a day.
Kensington Palace generally gets poor reviews.

Just a couple of follow up questions!

Good Day! Recap: Staying April 30-May 8. Staying at Westminster Curio London (Hilton). Couple of logistics questions:

1. The Uber boat. Is that the ONLY boat, really to take? It sounds like that's the "main line" so to speak. We are taking a river trip to Greenwich for a day, and are assuming we need to take that particular boat but I'm hearing about "hop on hop off" trips, etc. Just need a little clarity. I'm probably overthinking it.

2. We're heading to Windsor on Friday (Castle). Looks pretty straightforward (tube from Pimlico to Paddington, then take the train from there to Windsor with a change in Slough). I have the citymapper app and I think I've got it figured, but is that train one that goes to Windsor every few minutes? I just want to be sure we don't get there only to have missed it and have to wait for 40 minutes.

3. Double Decker Bus. We have two options we're mulling over. 1.) Day 1 hit the ground first thing with a "topless" DD bus tour (HoHo). or 2.) Take one of many "tourist friendly" routes on the regular city owned double deckers. Any advice between the two would be great. Also, I'm assuming there's MORE than one company that offers the HoHo. Any advice on which one to choose if we go with option #1?

Thank you all in advance!

Double Decker Bus. We have two options we're mulling over. 1.) Day 1 hit the ground first thing with a "topless" DD bus tour (HoHo). or 2.) Take one of many "tourist friendly" routes on the regular city owned double deckers. Any advice between the two would be great. Also, I'm assuming there's MORE than one company that offers the HoHo. Any advice on which one to choose if we go with option #1?
Thank you all in advance!

Option #2. I can’t stress enough how disappointing, frustrating and slow the London tourist HoHo buses are (and I love them in almost all other cities).

I realise I’m repeating myself above! Last time I did one with my teenage nephew visiting from overseas. He really wanted to do the HoHo but ended up pleading to get off as it was so frustrating.

As regards boat trips the Uber boat is the commuter boat essentially which is part of the London Transport system- pay for with Oyster or contactless as you would for a tube or a red London bus.
But not included in the daily/weekly fare cap.
No commentary. The beauty of it is you can join it at Millbank Pier (straight below your hotel) every 30 minutes and stay on all the way to Greenwich, via various piers, hop on and hop off at will.
There are also various other narrated sightseer boats from bigger piers like Westminster and Tower.

There are multiple HoHo operators, including one which last year was advertising itself on its buses as American owned (I can't remember the company name). I doubt it matters which you use- all the companies use broadly similar routes.
The TfL routes are only tourist friendly to the extent that they pass the normal tourist sights. None passes all the tourist sites unlike a HoHo and of course no narration. They are at the end of the day just normal bus routes that happen to pass the tourist sites and most of the time will move at the same pace as the HoHo buses, there not being that many bus lanes in the centre of London.
And TfL buses of course stop at every stop- HoHo's only selected stops- personally I think it's swings and roundabouts.
Unlike many on the forum I'm relaxed at which you do. At least on an open top bus you are in the open air to keep you alert.
One compromise is to take the Routemaster T15 from Trafalgar Square to the Tower of London- a tour bus, but not at high HoHo fares, yet higher than it's TfL counterpart.

The tube from Pimlico to Paddington would be a change at either Victoria (onto the Circle line) or Oxford Circus (onto the Bakerloo).

If you change instead at Green Park onto the Jubilee line one stop to Bond Street you can join the Elizabeth Line there to Slough (rather than at Paddington).

The Elizabeth line train to Slough runs around 6 times an hour, that from Slough to Windsor and Eton Central every 20 minutes.

The tube from Pimlico to Paddington would be a change at either
Victoria (onto the Circle line) or Oxford Circus (onto the Bakerloo).

If you change instead at Green Park onto the Jubilee line one stop to
Bond Street you can join the Elizabeth Line there to Slough (rather
than at Paddington).

The Elizabeth line train to Slough runs around 6 times an hour, that
from Slough to Windsor and Eton Central every 20 minutes.

So it sounds like taking the Elizabeth line is a bit more schedule flexible? From personal experience, do you find any particular way of getting there to be superior? We're not super budget conscious when it comes to getting around. (My wife will rebook the same hotel room 25 times in 6 months each time she finds it's dropped in price, but once we're there, we tend to favor convenience and comfort over a few bucks). That said, is it ridiculously more expensive to just take a car or black cab? (Uber I believe is no longer an option?)

A single train fare using contactless payment (not an Oyster card in this case), will cost £7.70pp leaving after 9.30am and take c30-45 minutes depending on start point.
Any form of road transport will be slower and a great deal more expensive.

I'm not sure where John gets his fares from (I think he's given you Paddington to Windsor), TfL gives me Pimlico to Windsor and Eton Central £10.50 each way off peak, £15.30 peak.
Leaving the fare aside (it's a mere distraction) I would join the Elizabeth Line at Bond Street myself if going all the way by train, simply because at Paddington that saves crossing between extreme opposite sides of a big station.
Maybe I know where I'm going (I've been using Paddington frequently for over 40 years, long before the Elizabeth Line) so haven't noticed it but a few days ago on another thread someone said the signage at Paddington wasn't great. Maybe so, maybe not.
But what I would probably do, if it was me, is catch the #36 bus from outside Pimlico Station to Paddington. That will drop you right beside the elevators down to the Elizabeth Line Stations. Yes it's a bit slower, but then I like seeing the London views when I can, rather than tube tunnels. Personally that feels more convenient, the fact that it saves £1.05 is neither here nor there.
In London there are almost always multiple ways to do a journey, and it all comes down to preference. There is rarely a 'best' or even a 'superior' way as such.

We can only give you the obvious options, and there are other less likely options (Central Line from Oxford Circus or District Line from Victoria to Ealing Broadway, change there for Slough (in preference to Paddington, which I keep in my back pocket if there are disruptions at Paddington).

At the end of the day it's not the frequency of the trains to Slough which is important- there could be one every minute- but the frequency of the shuttle from Slough to Windsor- at every 20 minutes. That is the pinch point.

"In London there are almost always multiple ways to do a journey, and it all comes down to preference. There is rarely a 'best' or even a 'superior' way as such."

I agree 100%- even looking at what Google maps gives you for public transport, you will often see lots of different options with various pros and cons.

As with many things, there's not a single right answer... tho there may be many wrong ones.

Such helpful perspective, thank you!