All the Colt Govt. Model (1911) pistols shipped to England before mid 1915 were in 45acp caliber and were commercial serial numbered guns with the 'C' prefix.
They were for retail sales and a great number went to Military personel as Officers (in WW1 at least) had the option of purchasing their sidearm from either 'military stores' or a civilian dealer.
Rule was 'caliber .455' at that time. But many disregarded that and bought the Colt 1911 45acp anyway.

Even Mr Churchill, called to active duty after resigning from the Gov't, purchased his Colt Commercial 1911 45acp for duty in France in 1915 as a Reserve Major.

The 455 S/L caliber 1911's started shipping over in 1916 for British purchasing contracts. They have 'W' ser# prefix.

Any firearm to be sold in England then or now needs to go through Gov't controlled proof. Additional reproof is necessary if certain types of work are performed on it before it can be returned to it's owner or resold.
Any surplus Military arms (sold from service) need reproof under a civilian set of standards and remarked as such before they can be resold.