I have a ticket to see Mamma Mia show @ 7:30 PM, but will do the following prior to the show:
Take Big Bus sightseeing tour and get off at St. Paul's Cathedral for 11:00 am guided tour, then head over to Tower of London, allowing about 3-4 hrs for touring, I don't necessarily have to see everything there. Then return to the Bonnington to get ready for the show. I must get to the theatre at 6:30 to pick up my reserved ticket I bought on-line).
Of course I will have to allow time for getting from one event to the next. Is this realistic?