I emailed the seller inquiring about this. They even have it posted under the firearm shipping policies on GB

'Antique' firearms need not be shipped to a licensed dealer. These can be shipped directly to the buyer. An antique firearm is a firearm built in or before 1898, or a replica thereof. The exact ATF definition of an antique firearm is:
Antique firearm. (a) Any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; and (b) any replica of any firearm described in paragraph (a) of this definition if such replica (1) is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or (2) uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade.