I belong to the ALA, as you could belong to the BLA Duck and Dive. I went to their national convention in Indianapolis this year. People there were from all over the US, Canada and even a few from your side of the pond. Needless to say, if you could afford to play, the auction at the end of that event was awesome. I brought Xenotoca eiseni and Xenotaenia resolanae to sell and other people brought what they had in surplus. The auction is really our way of raising money for the club and spreading around very nice livebearers. I even saw some beautiful lyretail red albino swords that looked just like the one on the cover of the livebearer edition of the Tropical Fish Hobbyist, the April 2009 edition. I think the reason it looked like that one is because it came from the same breeder who was selling off his surplus at the auction.
At that event, I am a beginner only. In your position, I would connect with the British Livebearer Association, I may anyway, to get the fish that you are lusting after. Any fish that is available eventually shows up at one of these events and if you know what you are looking for and are willing to spend the money, you come home with them. I actually spent more on plants and fish at a recent local club auction than I did at the ALA auction.