Pretty sure you can buy Tower of London tickets from underground stations, although there is never a queue anyway. Would also be very surprised if Westminster Abbey ever had a queue.

The places which always have massive queues are the London Eye, Madame Tussauds and the London Dungeon. I think you can get fast-track tickets for these attractions if you buy in advance - check the websites. Although why you would want to go to any of these three places is beyond me - even the Eye is a bit rubbish. Technically brilliant, but London is the flattest, dullest city on earth from the air!