It's perfectly normal to crash on a mate's couch or floor when you arrive in London, but you don't want to overstay your welcome.
You should chip in at least £5 a night (to help pay for communal things like loo rolls etc) and you should obey the Dossing Rules. Learn them by heart.
The Dossing Rules
1. Don't wait to be asked to clean something.
2. If your rubbish won't balance on the pile, take out the garbage.
3. Don't phone home without asking.
4. Don't look at porn on the computer.
5. Always make sure the housemate gets the phone messages you took when they were at work.
6. If you can't cook, at least buy the ingredients.
7. If you won't buy the ingredients, at least buy bread and milk.
8. Never drink beer unless offered. Beer always belongs to someone else.
9. If you're sleeping in the loungeroom, you go to bed when everyone has finished watching TV.
10. Never take charge of the remote. It always belongs to someone else.