Timing of afternoons in London

British Museum will still have a line for security. Don't bring a backpack or large purse & you'll be through it fairly quickly. Whether you walk or bus/tube, I wouldn't book entry before 5:30pm. Don't forget, you'll have to reclaim your personal items after the Tower tour.

Take a look at Google Maps to get an idea of travel time. For public transportation just plug in the day of the week and approximate departure or arrival time to get an idea of schedules for Aug/Sept.

Before heading to London, add the Citymapper app to your phone. It will tell you which tube/bus line to take, when to get off, what exit to use. Also good for walking directions.

Buckingham Palace tour generally takes 2-2½ hours plus security line. The only thing I know about Harry Potter Studios is book ASAP. They can sell out quickly and many posters come here sharing their disappointment.

Sunday roast in the UK is not a quick meal and should be reserved in advance at the better places.

Re Guards: IMO the Changing of the Horse Guards is a different, but better experience. It is not mobbed like the Palace Guards.

Best time for Westminster Abbey is first thing in the morning. Book the Galleries ticket for 30-60 minutes after your Abbey entrance time. If you want to take the Verger Tour if available, after entering the Abbey head immediately to the counter to purchase the tour tickets that work best with your schedule.

Tower of London and St. Paul's is on the same side of the river. Assuming you will spend the morning at the Tower, I would book St. Paul's in the afternoon allowing for a lunch stop in between.

Again, use Google Maps and Citymapper to check your travel times.

For the Tower of London, I recommend going straight to the Crown Jewels upon entry, and then going back to the front after for a Yeoman Warder tour. I think we were at the Tower for about 3-3.5 hours and then walked over Tower Bridge. I'm assuming you will want to eat lunch, so I would something after walking over the Bridge. After lunch, you can walk along the river to Millenium Bridge which will get you to St. Paul's. I find that walk really pleasant. Not sure what time the last entry to St. Paul's is, but I wouldn't book something before 3 pm.

As already mentioned, book the Harry Potter tour as soon as you can as the slots sell out. The studios were amazing so if you are a fan it's a must see. It takes over an hour to get there and then another hour and change to head back to London afterwards so be sure and factor that in your planning.

You could always have the roast lunch in Watford. A local article says that the best in town is the Prince George pub, which by chance is very close to Watford Junction station.
Currently all slots at the Studios are available after 2pm that day until 6.30pm.
So if the Palace ends at 12 noon, take the Victoria line tube to Euston (10 minute journey).
You will make the 1252 train with ease, arrive Watford Junction 1312.
The big drawback is that 1st service ends at 1245, and second service starts at 3pm as tables are booked for 2 1/2 hours.

For Windsor on the Saturday, trains leave from Paddington and Windsor.

For 10am at the Castle the latest possible connection from Paddington is at 0908, so leave Victoria on the circle line tube no later than 8.30 am.

There's also a Changing of the Guard at Windsor Palace.

I have made my reservation for 3 pm for harry potter studios, we will skip trying to find roast lunch since timing looks to be an issue. The studio is open until 10pm so that should be more than enough time.

I see the 9:08 train from paddington that we will plan to be on, so we will be at victoria station by 8:30. We are planning to see the changing of the guards at 11 as it is scheduled to be done on Saturdays.

Thanks for all the help.