oooooooOOOOOOOHHHHHHhh! You lucky duck. I don't know of any SF stuff to tell you about, but I remember loving Trafalgar Square and feeding the flying rats. Fish and Chips was the only good food to eat there. I mostly lived off of tea and pints and chocolate while in the UK. Go to Scotland, it's ten times more beautiful than England. Edinburgh is lovely! I enjoyed getting lost in (I think it's called St. James Park) in London. And the tube is much fun. Eat German chocolate Kinder eggs for me and Cadbury Crunchies. YUM!. Guinness tastes better in the UK. I can't drink it here, it's bad. I don't know if you drink now or not. There will be lots of pretty flowers to take pictures of duh! Have an excellent time!

The best pub in all of London for a good weather evening (in my humble opinion) is one called the Pitcher and Piano, out in the glorious and non-touristy, yet fully public transit accessible neighborhood of Richmond. The pub has a huge outdoor deck right on the Thames overlooking the water, a small bridge and a riverfront walk, and is THE place to hang on a weekend evening. It is a very fun crowd, and they serve all the pints and pimms you can order. Food is reasonably priced too. Richmond itself is filled with parks, quaint shops, riverside bikepaths and beautiful homes. If you're looking to make a day of it and have the 12 quid to spare, you can visit Kew Gardens out in that direction in the same day.
Best of luck to you, and whatever you do while you're there I hope you enjoy your trip!
Cheers,
Ali