For travellers in London with a more "realistic" budget, if you go to the area near King's Cross tube station, go inside any pay phone booth, and you will find the inside covered with postcards with escort ads. The reason they do this is because most of these operations are within minutes from where they post their ads. Most are independent SPs, but some are small scale agencies (i.e. 1 or 2 SPs). I visited a asian SP there earlier this year, a really cute Japanese student (easily a 9+ in looks and body by Toronto's standard) and I paid 60 pounds for 1/2 FS (roughly $150 cdn) and it was very nice - TOG nice, except everything was safe. They have a really neat system where once you enter the bedroom she hands you a real "menu" just like a restaruant, with a list of extras (e.g. for bbbj, +20 pounds, for bbbjtc, +40 pounds, etc.)
It's pretty cheap for London standard, I guess. And certainly within most people's budget. But if you want "cheap" and "quality", nothing beats EE or Amsterdam. That's another story