(1) Prostitution is *legal* but pimping, public soliciting, brothels, and other organized forms are not.
(2) Avoid clip-joint-type clubs such as those in Soho. Enticing women at the door will promise anything, you will be seated, served a drink minimum, perhaps have some girls sit with you and ask for a drink, and then you will be presented with a bill for perhaps 100's of pounds. That bill will be enforced by huge bouncers who enjoy beating up those who protest. Cops will not assist you in this kind of situation. Just don't go in.
(3) Walk-ups, however, are generally safe. Typically a simple note will be pinned to an apartment walk-up door for massage or the like. Go upstairs and one or possible two girls will be there.
(4) For a good site see
http://punternet.com.
(5) If you are into more of a party or group scene check out
http://www.ladymarmaladeparties.com/ and
http://www.pamsparties.co.uk/index/ . You won't find the "top toddy" there...more like average girls or MILFs...but the price is very reasonable, MSOG is typical, and it's a scene unlike anything you will typically find on this side of the pond.