Have you come across http://www.uk-poly.net/
yet? It's still a bit slow to get going, but at least it's UK-based.
I suppose that the flipside of what I wrote above is that many people have had their heart's fingers burned in relationships by people who leave them for someone else (or they've seen it happen to others) so it's not surprising if folk are wary of the authenticity of "no, I can love you AND person x equally". Perhaps as time goes by and there are more role models out there for successful poly relationships, people will be more willing to trust that it's just as likely to work as the monamory option (which has a crap track record but a huge propaganda engine in its corner)