Ignore the doom mongers.

I've got sweet FA and there is loads to do in London that doesn't cost that much.

Rule number 1.

When you arrive go to a news stand or news agent and Buy a copy of TIME OUT magazine.

Accomodation ia the stumbling block from my angle as I've never had to find accomodation for a short stay in London.

Perhaps check out the lonely planet website or the "rough guide" website.

If that doesn't work, nip down to a local bookshop and buy a copy of the lonely planet guide to London/Britain or whatever or the rough guide.

Perhaps whilst comparing them to decide which one you want you could take note of a couple of hostels before deciding that you don't want either ...

good luck.