Klivie Hi Klivie, welcome to the forums!
Quite a few users have built SEQ v4+ (and even MatriCes 🙂) in the U.K., but i think most reverted to Mouser, as it comes with free shipping and i think with fully paid VAT/Tax duty for the U.K.. On the other hand, the SEQ v4+ uses many standard parts like resistors, connectors or the large 1206 SMT parts like shift registers, that you should be able to source from any U.K. based parts distributor, it’s potentially just a bit more work finding everything.
Best regards and enjoy!
Peter