Need help creating a London Itinerary 3.5 days:

If you go to SoHo grab lunch at Mother Mash (make a reservation). Consider walking around Hyde Park, there are red collar parakeets (they are very large) flying around. It is a lovely retreat from the bustle of the city. You could also take a train to Brighton for the day.

You have a very short stay, so you will definitely need to trim your list. Given London is a big city, it makes sense to group sites together geographically to save travel time.

Buckingham Palace, Houses of Parliament and Westminster Abbey are near your hotel in Westminster, so you can do them together. Marylebone is easly walkable from Westminster, as is Covent Garden, Chinatown and Soho. Likewise Harrods and the Natural History Museum are both in South Kensington.

A few other thoughts - personally I much prefer Fortnum & Mason and Liberty to Harrods. Either way, both are worth visiting - Liberty is a unique building.

Oxford - can easily be done as a day trip BUT you have such a limited time I would advise just sticking with London.

Museums/galleries - you haven't said what you're looking for, so it's a bit hard to advise. My favourite is the V&A (also South Kensington.

Changing of the guard - every Monday, Wednesday, Friday and Sunday. Regiments are mostly drawn from the Household Division, but occasionally others - we were in London earlier this week when the Royal Gurkha Signals was the regiment on duty, and the week prior it was the RAF. Occasionally it's the Australians or Canadians. It gets very busy and can be difficult to see - you might want to consider the smaller ceremony at Horse Guards instead- you can stand right up close if you approach from Horse Guards Parade, as it's not behind a fence. This is normally the Household Cavalry. There is also a guard mounted at Clarence House, the residence of the King and Queen, but no ceremony there.

The schedule is here: https://www.householddivision.org.uk/changing-the-guard-overview

A few other thoughts - personally I much prefer Fortnum & Mason and Liberty to Harrods.

I agree. And Harrods is outrageously expensive. I really enjoy wandering through F&M, and they have a lovely restaurant called The Parlour, which you can get a nice cream tea.

^^^ On the other hand, if this is the OP's first visit, Harrods has the name recognition and is fun to wander. If the outing is primarily a wander through, not a serious shopping, Harrod's may better fit the bill. I have taken first timers several times and they generally quite enjoy the food halls: https://www.harrods.com/en-us/departments/harrods-food-halls Fortnum and Mason would be a fine substitute for that.

I personally shop in Liberty, but my hubby and sons don't share the enthusiasm. No problem getting them into Harrods for a brief round.

Or - try walking through the Burlington Arcade for window shopping rather than a department store stop. https://www.burlingtonarcade.com

OP - can you define your interests more? History? Art? Iconic London sites?

If you are interested in wandering the streets, there are several online walking maps and apps. Or take a look at the guided tours offered by London Walks. Several of your sites can be grouped and included in a half-day or several hour walk by. Similarly, the Thames boat tours will give views of Big Ben, Tower Bridge, etc.

There are such differences between the Natural History Museum, British Museum, and National Gallery that it is impossible to say which is "best" - at least for me. Take a look at the websites and see which best suits you. To manage your time, identify which exhibit or paintings you want to see before arriving. In my personal opinion, the British Museum is unique in the world, the National Gallery's collection has a number of well-known works, and much of the Natural History Museum is a fine presentation of their collections. (the latter can be over-run with kids in my recent visits with the past 5 years.)

With 3.5 days, I personally would skip Oxford unless it is a personal interest above London's offerings. Have you considered Hampton Court?

Just one idea for you to consider. We were in London last May and went to the Sky Garden to get a view of the city. It turned out to be a real highlight for us! Look up Sky Garden and either try for the free tickets that I believe are released 3 weeks in advance and get snatched up instantly, OR make a reservation at one of the restaurants or bars to gain access to the garden and viewing area 36 stories high. We didn't know about it until the freebies were already gone, and reservations were fairly full. So we ended up reserving at Darwin's Brasserie for a breakfast at 8:30, and it was marvelous! We had a gorgeous clear morning, riding up the elevator for 35 floors, a lovely breakfast with a view, and then we could have stayed up there as long as we liked. Evening time would be a cool view as well with the city lights below, but we just loved the indoor garden area and looking out at London from there.
Enjoy your trip!

-Sue and Scott
(also enjoyed Borough Market and Westminster Abbey and would recommend attending the Evensong there - don't have to be religiously inclined to be struck by the beauty of voices singing in that space. Look up times and go early to line up to attend - by 4:30 at least.)

only a quicky - for spelling and pronunciation when you're in London, the place in London (formerly (in)famous for strip clubs and strange films) is all one - Soho - with all lower case except the S.

The part of New York City in Manhattan which is South of Houston Street is SoHo. If you spell it that way in London people may be confused...

Just a tiny thing.....

I concur with Simon's post and itinerary and this is exactly what I suggested to a friend who went for the first time with her teenage son at the beginning of May. What is best depends on who you are travelling with and whether you stick together every minute.

Don't bother with a trip our of town. Save it for your next trip when you will plan for more time.

National Gallery may be a nice follow up to what you saw in Paris. We always go to the Impressionist rooms and sit and sketch when we visit. Been doing this for when my daughter was 5 to 15 years old.

Don't bother with Harrods. Go to Liberty as it is a beautiful building which is different from what you have where you live.

Walk around Covent Garden, Soho. Chinatown is one main drag. Go to Neal's Yard to the wine bar. Best wine and cheese menu and a lovely spot.

If you have been travelling for a while, you may wish to slow your roll and enjoy.

Don't bother with Notting Hill unless you particularly want to see it. It's okay. It probably better when the market is on and an absolute scene during the Carnival.