If you haven't already done so, check out TNT Magazine. It is produced by a mob called Australian ExPress, (get it - duh!). It is availabble free at a lot of London railway stations and ex-pat haunts. Kings Cross has a big population of Aussies, Kiwis and Boks.
Has loads of flat shares, and crashes. You'll get an idea of the costs you'll be facing.
London has it's fair share of crime and typical city troubles, but I always thought it was quite a good place to be. Mind, I liked Sydney as well.
A lot of the "great historical objects" are pretty disappointing when you get to see them for real. Your imagination builds them up, the reality is often a let down. The pyramids at Giza, Egypt are pretty impressive for their age, as are some of the Inca constructions in Peru, (a lot newer of course, but impressive workmanship).