You have not mentioned where exactly in UK you reside but I know of this International project which deals in the publicizing and distributing rare varieties of fruits by amateur and professional breeders. They supply these fruits including crab apples to every part of UK. If you'd mentioned your whereabouts I would've been able to direct you to a retailer in your area but since you haven't I'll give you the link and you can easily find and contact one. Here's the link:
www.nuvar.net
www.nuvar.net