Hi rubric, Gumtree is a good start. You will obviously have to filter out the legit shops. A starter for 10 is to look for shops which repost their adverts regularly (like the Chinese Health Centre in Cricklewood) or have a mobile phone number as contact within the body of the advert. An initial text asking for types of massage / prices to break the ice followed by one which asks if they do anything else (I don’t mention the word extras). The legit shops will always reply with a definite no or just list another load of treatments, the ones where there may be something else on offer just reply with something like "we can talk when you come in". Very rare that the shops you are looking for will actually text back with anything mentioning extras.
Happy hunting! oh and make sure you post any reviews on here.