Around the city centre I've been to two places so far, Sawadee Thai Massage and ATT (A Touch of Thailand).
Sawadee is located next to 168 oriental on Nelson street, go into the multi-story building and all the way up the spiral stairs. I'd say their service is usually excellent but very inconsistent regarding happy endings etc. etc. The manager seems to be permanent but the masseuses switch out quite often and depending on who you get, the price and "extras" on offer may vary. I'd say its about a 50/50 chance that the lady offers a happy end... if she asks you to keep your undies on you can be quite sure there's nothing extra on offer. Despite the caveat, the actual massage quality is excellent so if you're willing to risk not getting off I'd say it is well worth it. I've had several normal massages at Sawadee before I was offered HE for the first time... take from that what you will.
On the topic of HEs, the basic one starts at 20-30 quid extra and always is done with great technique. If you want no clothes it gets
very very expensive with some people asking for an extra 80 pounds. Full Service or Oral are a big no no here, you won't get anything more than light touching.
ATT is more dingy, on Denmark Street (if you're self conscious about being seen this is not the address for you) and you have to rely on someone manning reception to let you in as the front door is always locked. The massages on offer are decent, but not great, but at least you're always guaranteed to be offered a HE and a good shower afterwards (they use a LOT of oil). Extras are slightly cheaper, someone on this forum mentioned having a FS here but in my experience this has never been offered or hinted at.
Neither of the two places offer a B2B massage so if you're looking for that kind of slippery fun I'd recommend AW instead, if you see a masseuse like Blue Sienna or Sensual Kate available book fast as their schedules get very full very quickly (for good reason
)