Can you tell a bit more about your concerns?
Have you already found someone that you are planning to meet or are you asking about how to hook up with someone?
If you already have found a third person
or a couple that you don't know since before we would probably prefer booking a hotel room. Then meeting up in the hotel bar. If you feel there is good chemistry continue up in the room. Otherwise just say goodbye and no harm done.
If looking to meet someone.. If a couple we found going to swingers club and just socialising and experiencing the atmosphere was a good start. Then it kind of comes naturally from there. If looking for an extra guy then not sure what the best channels is in UK but either here or another forum.
Good luck and feel free to send a message if want to talk more.