Bulgari Hotel London

If you ask Londoners who actually use hotel spas where to sweat, a lot of them say Bulgari, and they're right. The spa here is a genuine destination — a 25-metre pool clad in dark wood and gold mosaic, a gold-leaf vitality pool, and a hush that the Knightsbridge crowd takes very seriously; sauna heat currently comes via the bookable Recovery Suite sessions, so plan for that when you book. It feels less like a hotel amenity and more like a private Italian wellness club that happens to have rooms above it. And what rooms: deep, dark, silk-and-marble suites that are among the most comfortable in the city. It's expensive, obviously, but the spa alone justifies a lot of it. For the best all-round hotel sauna in London — atmosphere, facilities, and pool in one — this is the benchmark. Guest access is generous; book treatments ahead as they fill fast.