As an astronomer, perhaps a trip to Greenwich Royal Observatory?
Food wise:
Decent pub grub. Ask for local advice in whatever area you're in, otherwise you'll just get standard chain-pub fare.
Nothing more British than Indian food
There have been some suggestions already, but there are a couple of areas where restaurants are wall to wall (Brick Lane, for example) and you can't go wrong.
Fish and chips? Best places are the ones that cook more to order. Look for one with a modest queue.
Aside from that, you can have probably absolutely anything you can think of. Thai, Japanese, Chinese, Korean, BBQ, Lebanese, Italian, French, Spanish, Steak, you name it. Too much to sample in a short trip.