Hey! As ive recently been doing the same A-Level, my best advice is to walk, ALOT! I spent about 7 days walking (one day a week) it took aged, but I wondered across london for some good pics. That was for an "architectural photography" project. It really did work, but it was quite hard, walking about with a 35mm and a medium format camera. Plan the walks though! Say that your going to look in specific areas and have a plan, or you end up spending hours walking a few hundread metres!
As for places, hmm, I would suggest firstly, the thames! There are really good spots from Tower Bridge all the way up to the Dome, I am not too good with that bit of london though, so I don't know exactly where! A good way to see it all is by taking a boat ride actually, but you would need a big zoom lens!
Also, surprisingly, Hampstead in N London is really good. There are 2 churches that are almost in ruin (one being done up, and the builders let me into the grounds/dorway to photograph it!) There are also alot of other buildings hidden around the small lanes that need repair and look quite good too, but again Walking is required lol!
Good luck with the project, I found it hard but really fun!