There's lots of great SG girls here. Even though you're travelling there alone I'm sure they can fill up your itinerary quite easily

Enjoy your time whenever you do end up going. I've been to Canada and Mexico but never across the pond... yet.

September may still be busy (to a point) due to the Olympics.
Accommodation may still be on the high side too, but should be so much better than July/August.
I'd maybe look at airbnb for prices/ideas for rooms?
London I'm sure has many a good hang out..... Come down to the coast (Brighton) for the more diverse surroundings..... He'll they call it London by the sea!

Although the Olympics will be over the Paralympics don't finish until the 9th September - so expect overpriced accommodation until then. Apart form that September is usually nice in London, we can get a nice spell of weather around then. The hostels tend to be centred around West London; Earl's Court and Shepherds Bush due to the large Australian/South African communities that gravitate there. Sorry, I can't recommend anything specific, but hostels are great places to meet travellers who you can explore the city with.