1. Rios Naturist Spa, 241 Kentish Town Road, London NW5
(Has basic private rooms to play with other guests or any WG you take in. Couples only on Sat evening - lots of action, including the large hot tub. Other days/times a bit of a sausage fest in my experience and chance of action very low with other guests. Supposed to be naturist only Mon - Weds but managers often don't enforce it. Have often see girls concealing bikini bottoms with a towel round the waist. Outdoor smoking area. No idea what is on offer from the massages that take place in the basement.)
2. Naked Hands Spa, 14 Palace Gates Road, London N22
(Very small compared to Rios but nudity is strictly enforced - there is a prominent notice by the reception window about no swimwear. No chance of any HE from the therapists and you will be told to leave if you start any 'action' with other guests - no private rooms. Staff do use the facilities naked though whereas bar staff are only topless at Rios. I find it a nice quiet place to relax or before moving on elsewhere for a punt.)
I don't know Essex but if you can get to NW Kent then you might try either Silverleigh Naturist Spa or Eureka Naturist Spa - at both places you can shag a fellow guest or a WG you've taken in. Note that both will require ID and proof of address as they are private members clubs, although you can pay on a day rate basis.
