Well, there certainly have been fewer decal sheets aimed at specific Commonwealth vehicles than German vehicles, to be sure, but since there was a relatively limited number of combinations, most of us Commonwealth modellers have long since learned to cobble together correct decals from various sources. It doesn't hurt that, apart from teh formation signs, Commonwealth markings are vey simple and can usually be created with a bit of masking tape, paint & some lettraset white numbers.
More recently it has become possible to model almost any vehicle in NW Europe, the Med and Italy through the production of various decal sheets although the CBI thearre is still sadly under-represented.
As to the Jeep-type vehicles in service in the Commonwealth, the Willys MB Jeep far outstripped any other variant in availablility. There were only ever a few Bantams supplied under lend lease (only 3000 Bantams were ever made, 1500 went to the USSR and not all of the rest went to Britain) so after 1941, the vast majority of the Jeep-type vehicles were, in fact MBs or GPWs.
Paul