What an awful situation to be in
It's really all a big mess from a simple mistake and it seems as if Buckingham Trading Standards are trying to save face by not backing down.
I think you've hit the nail on the head there, it's either lawful or unlawful to use vaccines purchased abroad. If it's lawful then I can't see how Trading Standards Buckingham can continue to hold your kitten.
I think Jasmine's suggestion is an excellent one, get the press involved - if you have kids or grandkids (or any child relatives!) then even better, poor sad children missing their kitten 'stolen' by ridiculous double
trading standards.
I really hope you can get this resolved and soon, for the sake of the poor kitten stuck in quarantine when she should be with her loving new family if nothing else
Just out of interest was this a pedigree kitten you purchased? I'm guessing so as it would be unusual for back street breeders knocking out moggies to care enough to vaccinate them.