Firstly, many asian massage places provide far more than just a rub - they are actually brothels in disguise! Not all, but a good number of them. The walk in option is quite interesting, being presented with a selection of girls to choose from, just like that scene from Rush Hour. I haven't visited one of these places in a long time and i've no desire to set foot in one either.
Whats on offer is fairly straightforward. £40~£50 for a 1hr legit massage plus £20~£30 for a hand job, and an additional £20 if you want her to strip off & do a body to body massage for five minutes (B2B). No such thing as tantric in these type of places other than additional money for the same thing. 4 hands just means two massueses, and Nuru is an alternative gel to baby oil - its more slippery and more expensive. Its very unlikely you will get authentic nuru gel.
Best advice is to just walk into one, go for a 30min normal massage which should only set you back £30 and see if any extra services take your fancy. In Kings Cross there are a couple of reviewed places i) Kings Cross Beauty 319 and ii) Bamboo Natural Therapy. Googling the titles will get you the phone numbers and locations. I haven't personally visited either of them