I think that activity like this is still covered by the Sexual Offences Act of 1967, but this has been modified by more recent legislation that says no offence is committed if it would be reasonable to expect that no other person could accidentally happen upon the scene. It is a grey area, and an extension of the "privacy" idea without the law actually saying exactly what the circumstances might be.
So, this
could mean that in certain members only environments where all people present were well aware of the possibility of male gay sex taking place no prosecution would ensue. However, there is a specific exclusion in that a public toilet can not be considered such a place, even if the act between two or more men takes place in a locked cubicle. There is a specific offence concerning public conveniences with its own penalties.
See
here for the full text of the Sexual Offences Act 2003.