Maybe here:
http://www.tasteclub.com/tasteclub/f...ally-good.html
Do a google search with the keywords "butcher" and "offal", but you must select "advanced search" first and select only UK results in the "region" dropbox.
To avoid shipping costs, maybe add an additional keyword like the name of your town.
Or you can try calling some butcher shops near you, and ask them if it's possible to order bull or pig testicles.