In the UK is it acceptable for a lecturer/professor to join student societies?
In essence there is no problem, here in the UK, for the lecturers/professors to join societies. I experienced it first hand, both as a student and lecturer, and loved it. But you have the following ethical responsibilities: 

Socializing and Not Dating: Most social clubs go out a lot. For example, they might go to some other part of the country to do hill walking, or pub crawling which involves loads of drinking. The bottom line is that you can socialize with students, but don't lose your mind and start dating a student while camping somewhere. Some social clubs are hubs for dating, so join them for the cause and socializing, and avoid dating altogether. 

Close Friendships: Here in the UK, in my experience at least, some students are looking for an easy way out of studying. They are young, and therefore they just want to get by. Don't create a situation where they can take advantage of you and abuse your friendship.

Overall, know your limitations. Deal with students outside the university in a respectable manner, and don't get too comfortable. At the end of the day, you are the lecturer and they are the students.