Re: COLT .455 BRITISH 1911
I don't see any British military proof marks or a broad arrow, so it does not appear to have been in British service. According to Clawson, out of 17,500 Government Model pistols shipped to England, 600 were on commercial sales; that appears to be one of them and thus quite rare. I don't know the current laws about marking guns in single gun imports. Prior to 1968, that gun would not have been marked since it is of U.S. origin. And I have been told that a special request can be made to forego the marking on single collectors' items but I don't know how that works.
Just FWIW, that gun differs in some subtle ways from the standard government model. The magazine well is wider, the magazine is wider (the .455 magazine will not fit a standard .45 pistol), the barrel hood has a special counterbore for the .455 Webley Auto rim, and there is no barrel shoulder.