Hi Claudia,
from what i've read on the forums; flat sharing is considered a brothel and is illegal. however, my opinion is that if it is done correctly; that if you find a flatmate that you click with and can trust; you can work out a situation that is safe and sustainable. the key elements here are finding someone you can trust and keeping a low profile; as in only seeing a few clients a week. perhaps meeting your neighbors and having a back up story would help.
i can't help you with locations in london, as i'm in paris at the moment, i don't know london that well, but i do plan on relocating to london in a few months.
the most important advice i can offer is this: save your money. i know it will be difficult to not go hog wild and blow your cash on shopping sprees and vacations (like i foolishly did), but trust me, you've got to save your money. and pace yourself; work for a few months; then take a break and only see regular clients. you will burn out and it will be difficult to keep your wits about you once you burn out.
also, i have not done this yet, but check with penny about getting a WG buddy. it helps to have someone to talk to; someone you can trade stories and complaints with.
so, that is all i can tell you for now. perhaps if you give me more information about yourself, such as where you want to locate to; your experience; your situation; etc. i can give you more advice. i've been a WG on and off for about 12 years; worked in the States and in Europe. believe me, i've seen it all and been through it all.
good luck.
peace,
angela