Here is a comprehensive list based on visits and other reviews:
1. Oasis Sports Centre - near Covent Garden. The king of communal showers. Wetside mens changing room features 8 communal showers which open out fully into the main changing area. Towel hooks before you enter. Walkway to toilets and exits to pool go past the shower so plenty of people walk past. Although its a wetside changing area, nobody showers in trunks. Some even use the urinals completely naked. A very homoerotic experience, but certainly not a gay gym.
2. Sobell Leisure Centre - Islington - About 4-6 open showers with one additional cubicle shower in mens changing area. Towel hooks on the way in. Gets quite busy in the evenings when 5aside footballers are in the changing room.
3. Marshall St Leisure - Soho - yet to visit, but has very good reviews.
4. Better Gym Holloway - Islington - A very small communal shower area. If full, it would be almost cheek-to-cheek. Never seen many using it.
5. Islington Tennis Centre (need confirmation) - Yet to visit.
6. Muscle Bull Gym - Bermondsey (Central) - Four open showers. There's also a mens sauna in the changing room. Attracts gay crowd
7. Cyclebeat - Lombard Court - Approx 6 very open showers in the mens chaning area to use after a spinning session.
8. Finsbury Leisure Centre - Islington - Seven open showers in mens changing area
9. Ringside Boxing Gym - Bow - Open showers in the changing area. They are also available for filmmakers who use communal shower scenes.
10. Flaxman Sports Centre - Lambeth - (needs confirmation)