Hello,
Punt guns (single & double barrel) are legal in the UK & still used by a very few fowlers.

There used to be a company called 'Adams Punt Guns' on the Solway who made them. Alan Myers probably can make one (you'll find mention of hin on a google search). They come up for auction on a fairly regular basis - theres one in this weeks Holts London auction.

Regards
Webley