http://www.ddas.co.uk/
http://www.ifocas.fsworld.co.uk/UKCounties/area/index.htm

Practical Fish Keeping Club Directory
http://www.practicalfishkeeping.co.uk/pfk/pages/list_clubs.php

http://www.aquarist-classifieds.co.uk/directory/48.php

Viviparous Forum
http://www.livingfish.co.uk/forum/topic.asp?TOPIC_ID=130

British Livebearer Association
http://home.clara.ne/xenotoca/

http://www.caerurfaas.freeserve.co.uk/

Fifteen Years of Snakeskin Topsword Guppies in the UK - Alan Charlton
http://www.fbas.co.uk/NEWSPAGE.htm

http://www.livingfish.co.uk/forum/groovy_guppies.asp

http://www.world-guppy.de/english/article1/topswords.pdf

http://www.aquaristsreunited.fslife.co.uk/Pan-British%20Groups.htm

http://www.pet-x.com/pets/livebearer.html

http://www.cbel.com/fish_and_aquaria_pets/

http://www.fishlinkcentral.com/clubs/Europe/England/

One doesn't have to join a local group (although my impression is that UK aquarists and pondkeepers are better read and more active organizationally than American aquarists as a whole.) Finally, the Christmas issue of Practical Fishkeeping came in this week. ;)

Local meetings will have very inexpensive fish going at auction. Often one can strike up an acquaintance with someone who can help.
Bigger events will have more offerings of fish. Because there are a lot more people there looking for fish, fish sale room and auction prices will be higher.

While top, bottom and double sword guppies are to be found in the U.S., I think those patterns of guppy get more respect in east of the Atlantic.

I hope those sources, check Leon's first, are useful.