Tower of London on a Sunday

The only problem I see, other than complete exhaustion, is that St James' is on the Circle Line, as is the Tower, and the Circle Line often has construction closures on Sundays. You're not too far to walk to the Embankment though or other location for a bus towards the Tower. You will be tired Teddy Bears as you try to limp around the Ripper walking tour though...

Sophie, I'd plan to do something open-ended on the first day after that long overnight flight, and save the Tower and the walk for later. Jet lag is real, and at some point you will want to just crash. If you don't, you'll liable to be out of phase the entire trip.

I feel exhausted just reading your plan for Sunday. The jetlag from a West to East flight just wipes me out. Then getting to a central London hotel after negotiating Heathrow. But you may be able to deal with this better than me. Dont know if you have been to London before but it can take a while to orientate yourself. Also Sunday is a day when there are closures to tube lines for maintenance. Would really advise staying close to your hotel - you can walk from there to St James Park, or to the Thames by Big Ben/Houses of Parliament - London Eye is across the bridge from Big Ben if that is your thing.

You aren't going to really see the Tower of London in 2 1/2 hours on a busy Sunday afternoon. I strongly suggest you rework your schedule. Try showing up Tuesday at 9 am and plan on being there until at least noon. The crowds will be smaller. You can see the crown jewels without a wait. Your Yoeman Warder Tour will be smaller. You will actually remember some of it afterwards.

From what I've been told by several people, the Jack the Ripper tour is a waste of time. We did the Tower after flying over but I take an Ambien and sleep all the way. Also did the Ceremony of the Keys the same day.
I don't think 2.5 hours is enough time at the Tower.

Play it by ear and see how you feel. Realistically, you might not get into your hotel till 3ish so you work your way to the Tower it might not be till closer to 4. With the cost, I would definitely want to spend more than 2 hours there! Either way, try to build time into your schedule so you can see it another day if Sunday doesn't work for you. It is a definite must see and make sure you take the free Beefeater tour.

I would give the Tower more than 2 1/2 hours...but don't worry about the walking tour, I do it all the time on my first day and I'm always fine. Not too tired, but it helps me not fall asleep by 7 that first night. That way I wake up the next day well-rested.