I looked into this several months ago for a Wilkes boxlock. Chuck Lutz at TR Imports provided the following information. Note that this excludes shipping costs, which can vary a bit depending whether other guns are being shipped over at the same time. Francis Lovel in the UK may be able to help with the shipping if your seller is unable to arrange that.
We no longer handle shipping arrangements, because experience has shown that you can nearly always get better rates and ship sooner if it is handled by the seller, but we can certainly import your gun. Our fee is a flat $95, and we will have to pass along the air carrier terminal fee of $50 and in your case, customs broker expenses, (because your gun is valued at more than $2,000, it requires a formal entry, the expense for which should be a little under $200). Finally, shotguns from the U.K. are subject to a duty of 2.6% ($72.80). In all, excluding shipping, your total expense on our end should be less than $417.80
British Sporting Arms in Milbrook, NY also provides importing services but their fees are a bit higher.
