Curley maple, and birds eye maple, enjoyed a degree of popularity in England from the late 1780s through about 1820. If you can find a copy of the catalog, look at the rifles in the Metropolitian Museum exhibit of the Clay Bedford collection of English firearms.
Besides, the purpose was to make something to sell to Americans, not Englishmen. By the turn of the century (if not the middle of the 18th century) most of the walnut used in England was probably American in the first place - in fact one of the first things the Ketlands did in America was to purchase thousands of acres of walnut forest. I'd be curious to know if Pennsylvania restricted land purchases to citizens at the time, as this may have had something to do with their naturalizing as US citizens.
As to British made rifles for the American market, I don't think that many were made. There may only have been a 100 or less - after all, at 20 to the case, that would still be 5 cases and its more likely they were only 10 or a dozen to the case. The Ketlands were wholesale merchants, not gunmakers in the American sense, so just about everything they sold was in quantity.
I suspect that the potential customers were just as happy, if not more so, with the locally-made product. Of course, depending on what they look like today, they could just as easily be accepted as cut-down American products. I have seen what may have been others. In fact, I've known about them from the documentary side for years but they are very elusive to identify, partly because no one is looking for them. I particularly like this one because of its un-British use of a flamboyant American-style patchbox and butt design but I have seen others that are thicker, heavier and a good deal more clunky.
Some high quality English rifles were likely also imported but these would have been indistinguishable from the home product so, short of a slam-dunk provenance, we'd have no idea where they were used. I know of an excellent P. Bond (gold touchole, flashpan etc.) single barrel fowler that was the personal property of a prominent NE rifle maker though we'd have no way of know this were it not still in his family.
JVP