This unsigned rifle appears to be an English sporting rifle that was made using a "Germanic" rifle barrel. Reuse of European barrels, especially Spanish ones, was fairly common in 18th-century England. The barrel's Continental proof mark is on the side flat near the touch hole and looks like an anchor superimposed over a ball or globe.
The walnut stock, brass mounts, and convex face lock appear to be English. The mounts are not engraved but are of the same design (casting patterns) as some used in the 1770-1775 period. The tang carving is very much like that found on fowling pieces and pistols of the period. Two brass washers were used instead of a cast side plate -- this design begins to show up on a few Georgian pistols as early as the 1760s.
The overall quality of the work seems more likely to be from Birmingham than London. Some that have seen pictures of this rifle think it could have been made for a gamekeeper on an estate.