The UK has been breeding mice for longer than anywhere else that currently exhibits them, and people are usually clamouring to import them from us! The UK is considered ahead of other countries in their selective mouse breeding, so as long as you go to a good breeder here, you really will get the best

As for importing from elsewhere, it is expensive and requires a lot of paperwork. I think it is also impractical into the UK as a 6-month quarantine is required regardless of animal - which means there is no point with mice.