Trying out saddles in the UK
I'm not sure about UK-specific swap service, but here are a couple of things you can try: 


Check with your local bike shops and see if they have loaner saddles. My fave local bike shop has loaners of almost every saddle they carry and a sign out ledger. You can sign out the saddle and leave your credit card info. If it's not returned within a couple weeks, they bill you for the full price.
Buy a saddle you're interested in, and carefully wrap a long strip of electrical tape around the saddle rails. The rails are thing enough that the width of the tape can pretty much go around. Then clamp them just firm enough to stay in place, and you'll find that after removing the tape, there are no marks. Voila, you can return it according to the return policy of most shops ("no marks on rails"). If the saddle comes zip-tied into a cardboard backing or box, just buy some dollar store zip ties of the same color to put it back in. I've done this with great success. Just make sure not to lean it against a brick wall or ride in your retro 70s punk studded leather bike shorts. 
Check your local bike forums/ club scene and see if anyone is willing to loan out a saddle you're interested in for a few days/ weeks (offer them a safety deposit).