Any suggestions for a good massage/spa place in London?
I haven't personally been but a family member who came to visit really enjoyed the spa at the Renaissance Hotel at St Pancras: https://www.stpancras-spa.com/
The easiest answer is to ask your hotel for their recommendation. Or if you travel at a style above the typical RS traveler, your hotel might have a spa. For that matter, every 5 star hotel should offer such a service even to non-guests.
You can get a chair massage at Walk-in Backrub. Locations throughout central London.