Just went to Fitness First Stand (Covent Garden) to check out showers & sauna. Visited 3.30-4.30pm Sunday (closes 5pm on Sundays)
The sauna is indeed located in the men's changing room, so men-only and pretty perfectly located: tucked away from the changing room, at the end of a row of showers. The showers are unfortunately not open, but have frosted/translucent glass on all sides, and also face each other, so you could leave doors open and look across.
In terms of what I got up to:
I went into the sauna, sat on an upper bench. There were already 3 or 4 guys there, all fairly good looking - one guy in particular was ripped, massive chest. I got the impression I might have interrupted something, so I tested things by spreading my legs to open my towel so my cock was visible to anyone who looked. I got plenty of glances, which I returned. Being stared at also turned me on, so i was rock hard in a minute, and this was clearly visible to anyone who looked. The guy with the big chest pressed his towel tight against his groin so the outline of his hard cock was clearly visible. Another guy positioned himself so he could flash his hard cock at me and only I could see.
Within 5 or 10 minutes (and some tentative looks/grabs of cock under towels), we were all stroking in the open. The guy with the big chest reached over and started stroking me, meanwhile the two guys on the opposite bench were stroking each other while looking at us. Soon enough we were all rubbing each other's bodies: chests, nipples, cocks, balls, even some reaching under to tease assholes as we stroked (felt incredible). We punctuated this with breaks in the cold showers to keep cool, and covered up whenever anyone new came in - but then got back to wanking. Eventually most of us came and went our separate ways.
I suspect this sauna gets a lot of this kind of action - particularly on weekends. Will definitely be returning!