Try
http://www.galda.org (which is not quite U.K., but the German representation of EUFALDA) and UKOMA
http://www.ukoma.org.
Be warned though, due to the current restructuring of the aviation industry and more sophisticated flight planning and OPS Control Systems, there is currently quite a surplus of dispatchers (also well qualified one's) around. Will not be easy to find a job (let alone a well paid one).
Good luck
fcit