The dogs that many people call Jack Russells are not a recognised breed rather more often a type of terrier cross. There are clubs for Parson Russel Terriers which are recognised and there are breed clubs for Purebred Jack Russells not recognised by the British Kennel club that keep a registry of some of the unrecognised lines.
I did a search on Jack Russel breed clubs in the UK
PARSON JACK RUSSELL TERRIER CLUB. Sec. Mrs R M Hussey Wilford. Tel No: 01905 821440