Someone at one of the forums I frequent was asking recently about cheap good accommodation in London. I've searched the forums but the thread seems to have disappeared.
However just in case the person concerned is here, I've got a suggestion. It's the Youth Hostel Association at www.yha.org.uk They don't just cater for young people, people of all ages go there. And you don't have to be a back-packer.
And if you want a guide for a day to show you some of the sights or just for some advice then pm me - as a Londoner I know it quite well.
Albi