There are a couple Facebook groups... sandboas anonymous and sand boa keepers (I think) and one of those also has a classified group on FB too. You might want to check there. I have 2 KSBs (anery and nuclear females) but am in the US, so they are readily available here. Some of the big KSB breeders post on those groups, so they may know someone in the UK. I know several Canadians post on those groups as well.
Good luck! I know it stinks when you see an animal you want and can't find it available in your area.