sound reasonable?

Ooof. Ok.
Strictly for London, you really only have 4 days and not 5. What you're looking at for day 6 will eat up that day. IMHO you have WAY too many tours and have left little time for LONDON. Personally, I'd scrap all of it but the Westminster and the evening food tour, and maybe add a day trip to Windsor Castle.

Maybe on your first full day, take a hop-on, hop off bus around the top sights

Over the remaining days:
Visit the Tower of London

Add in the V&A, Oxford Street and the Globe (since they were on the wish list) plus the Tower of London and maybe St Paul's Cathedral.

Take a walk around Hyde park and over to Buckingham; take a walk to Parliament and the Clock Tower.

Maybe ride the London Eye (not my favorite but it might be fun for the younger people).

Herding 9 people around will be a chore if everyone has a different idea of where they want to go so make it easier on yourself. Other than just a couple of tours, give them a list of things to do, with hours and addresses, a map, and tell them to have at it. A 12 year-old is not going to necessarily be into a tour of musical sites for groups/ musicians he/she has probably never or only vaguely heard of?

Definitely nix the flight across the Thames; to where? Just dealing with airports will be a headache, and there's enough to see in London itself!

The Emirates Air Line is a cable car across the Thames in East London. Not an aircraft. Maybe the young children will like it.

I've never done it (it is an advertising ploy by the airline with the same name) and the two end stations have Emirates in the name. It is far east relative to most other attractions, but not too out of the way for where you are staying, in zone 3.

Chippenham to Deptford is some train ride - nearly 2 and a half hours including changing from Paddington Station to Cannon Street stations in London, and with 9 people? ooof!

You're a braver man than I am Gunga Din.

Why Cardiff as it is so far off your core routes? Are you planning on driving from Teignmouth to Cardiff, seeing Cardiff & then going on to Avebury before ditching car & train back to London? Far too much! It is 134 miles from Teignmouth to Cardiff - most of it on motorways (interstates). I have done it in just over 2 hours - with no hold ups. Driving from Cardiff to Avebury would take about 1 hour 30 minutes - assuming no delays. (It would probably be better to dump the car in Swindon for your rail trip back to London - if visiting Avebury). If going to Cardiff, why not ditch the car in Cardiff and then train back to London Paddington and don’t bother with Avebury. (Trains from Cardiff to London take 2 hours)

Don’t forget that it is MUCH cheaper to pre-book any long train journeys about 11 to 12 weeks ahead. Fares & schedules at www.nationalrail.co.uk You would most likely be using this train company - with whom you should pre-book:> www.gwr.com

These places relatively near to Teignmouth are worth a day trip:> Totnes, Dartmouth > coast road (A379) via Torcross to Salcombe.
http://www.visitsouthdevon.co.uk/explore-south-devon/south-hams

On another day, these places in SE Cornwall:> Looe, Polperro and perhaps the Eden Project near St.Austell
http://www.edenproject.com

Laurel, You made me laugh. Generations have quoted that in our family! I thought it was just us. LOL

thank you for all the amazing information. i should have explained, i know this is a weird combination of plans

this is family - and adopted family. I've known everyone over 25 years (except for the kids who I've known sense birth). We're traveled to other countries together, but none of us have done UK

originally i was trying to go the "bucket list" approach - knock off "must sees".. what i realized, rather quickly, i there are more bucket items that time allows. and, from our past experiences - seeing something famous doesn't stick to us, like the experience. so i refocused on experiences

other than my tween, everyone is over 18 - the can take oyster cards and see whatever they want once we're in the city. i put together a handful of experiences for us to share

The horse riding through the moors is for my tween, She rides English, and this trip was taking her away from horses for "too long"

Cardiff is for the 4 Dr Who/Torchwood fans (I tried to drop it, but the consensus was its worth it even to see the Torchwood entrance and get a selfie - fine)

Avebury is a compromise - my niece really wanted to see stonehenge, but the time it takes (not to mention the reviews) seem to suggest Avebury will take less time and allow us to actually have contact with the stones - sold

The Devon custom dinner is being catered by our AirBnB hostess, who was a semi finalist on Master chef.. so its at our "home" and something unique. She's making "local" food

I picked Chippenham to drop off the car as its close to Avebury, and the car drop off is close to the train station - I'm open to an alternative location, absolutely!!! (side note that doesn't matter.... DH is going to spin off and take a train to Anderton's Music store in Guildford, as he's obsessed with them - darned musicians). I realize its a 2.5 hours train ride I didn't see another drop off en route. I'm totally open to options

My husband, who hates to travel, had one requirement (besides Andertons) - see famous Queen (the band) related sites - hence the music tour.. the secret food tour is only for the two of us, its a "date escape" in the middle of the trip.

Westminster Abbey is my one "must see"

Pride - I've been in several Pride marches in the states. when i let the group know London Pride was going on when we there there, 3 asked me to get us signed up. Honestly, I was going to skip it, but this felt like support from the family, so i made it happen

The ghost tour is the one that could probably be dropped the easiest.

The Emirates Air Line was a compromise - my niece wanted to do the London Eye until i showed her the Air line - it should give us a nice view over the Thames and be unique. My tween is up for the air line, but not the eye. But this is one of the "not booked" so some can do this (its pretty close to our airbnb) and others can skip

thanks again for all the advice, keep it coming