London is a metropolis. Many many massage parlors/agencies and independent massage therapists are available to book. Let me explain how it works in reality.
Massage parlor or massage agency definition:
1. An establishment that offers therapeutic massage.
2. An establishment that offers illicit sexual services under the guise of therapeutic massage.
A place where you get a massage but most commonly refers to a house of escorts doing sexual services in disguise of a legitimate massage business. Usually if you see a run down building in the shady part of town with a neon sign saying "MASSAGE", chances are its a rub and tug joint.
Any business displaying itself as a place for you to receive a massage, but most of them will offer sexual services... if not, then you can't be sure of that your 'therapist' is a qualified, experienced one or even if she is able to speak a conversational English. These 'massage girls' usually coming from poor countries to make money for a short period of time stay in the UK. That's why it can happen massage parlors are changing girls quite often, it's difficult to count on them you can get the same girl other time.
They are paying for the advertisements and providing marketing and reception for the girls - so you're not booking with the girl straight usually. They often send another girl with a similar look if you have selected one of them from the list on their websites. This business strategy doesn't build trust between clients and agencies and makes the genuine, qualified therapists feel bad as they often need to prove they're not one of the 'bad girls'.