We were very interested to read your journey into the naturist spa scene and glad you posted your experiences.
We've been visiting spas and adult parties for about 7 years now as a couple. BGHS is a good one but it's restrictive opening hours and the avid enforcement of no playing in the wet area has curbed our enthusiasm somewhat. But it's their club and their rules.
The thing about wearing towels in the rest of the club; I think it is more a convenience thing as you have to carry your towel with you. But it is only right when there is food being served and consumed.
Many naturist clubs have an identity crisis in that the owners would baulk at the suggestion swinging went on. Yet they wouldn't stay in business without it. Total denial.
As you might already realise by now if the door is open, or there isn't a door, the couple/s are happy for you to watch as long as you want. But always ask before touching. Never assume. Always expect the same from others. Our biggest bugbear is people talking around us or outside the room. Generally it is stupid joking or mudane nonsense. Totally inappropriate.
A couple of other clubs we would recommend: Eurekas (lovely in the summer) and Paradise Spa in Dagenham. Also Kestrel Hydro (near Heathrow) which we're hearing good things about but have yet to visit. All very well run clubs but don't think any of them have "couples only" times. You might need to check if this is an issue for you.
Finally, this is a no pressure scene. As Mrs Looker observed, after we first visited spas and parties,there is a far better protocol going on than in conventional environments. Because everyone is of a like-mind and little alcohol is bring taken.
Good luck for the future!