There was (and is) a distinction between an importer and a dealer so far as licenses are concerned, but most importers also have dealer licenses.
As for those conversions, they were done after the guns were sold out of British government stores. Most were done in England, by Cogswell and Harrison, which at that time was owned by Sam Cummings, who also owned Interarmco (later Interarms). The cylinder was not shortened to allow use of .45 ACP ammunition, it was shortened to allow use of .45 ACP plus a half-moon clip, since the rimless .45 ACP will fall into the .455 Webley chamber without it.
The sole reason for the conversion was to allow buyers to fire those revolvers since there was no large stock of surplus .455 Webley revolver ammo. A British officer issued a Webley Mk VI would be given one (1) twelve-round box of ammo and told to get on with the war. One (1) more box per gun was kept in unit supply, presumably to be issued when the officer brought in 12 dead Germans.
There have been reports of those Webleys blowing with .45 ACP GI ammo; Webley fans have vehemently denied that any Webley could possibly blow, but the fact is that the working pressure of the .455 was on the order of 12-14k psi, while the .45 ACP runs 21k psi, meaning that for the converted revolvers, every .45 ACP round is a proof load. I strongly recommend not firing standard GI or equivalent .45 ACP loads in those guns.