We will be visiting 11 museums, castles, the Tower and other sites. Should we make timed reservations individually for all 11 or is there one website, we can book all or most all visits?
Tickets are usually cheapest when purchased from the sight itself. It's possible that some of the places you want to see are not terribly busy and therefore will not really require you to get tickets well ahead of time. If you give us a list, we may be able to save you a bit of work.
Personally, I don't like to buy sightseeing tickets ahead of time when it's not essential, because those things are usually not refundable if you for some reason have to change your travel plans.
When is this for? That will change the answer for some places.
are these all in and around London, or around England? Or even perhaps Wales or Scotland? The actual list will help us help you and give an accurate answer.