I recently exported a shotgun to the UK through a licensed exporter here in the states. The gun was pre-1898, which the exporter told me the UK also considers it an antique. So it was imported as an antique.
It made it over there to the buyer with no issues. And to my knowledge he did not have to re-proof the gun. But, it was an english built gun and was originally proofed anyway.

B.Dudley