My lord am I talking to a brick wall here?
For a start, British units commonly used whatever they could get their grubby mits on, I mean it was an army in name only post Dunkirk.
British units commonly used US vehicles, some like the Grant sure saw some limited changes in design, though it should be noted that the British did also use Lee’s as well. As for the Sherman 2 and M4A1, its the same tank merely named differently. Like the British would go on to field the M4A2 sherman, should that also be added twice since the British used it too?
As you pointed out the main reason why British units didnt use US small arms (as a whole, there are exceptions to this rule which going by enlisted logic completely invalidates it, anyhow I digress) is due to the logistical issues of shipping forward two types of ammunition to the same regiment.
But where they did or at least could, as in the case of the grease gun for one such example, they very often used a mix of guns even in the same unit. Something that would also be seen in German units especially on the eastern front, some using a weird mix of italian, soviet designed, german and maybe even us lend lease weapons all at once.
However, ignoring the historical points of this completely and merely looking at the gameplay. And now remember your arguing from the point of view a new player being so unable, that they cant even find a folder accidentally.
You in turn expect new players to effectively double the amount of research that must be done. Again coming from the same person who thinks its too much to research a mere 30k to get the Grant to then get the Lee.
Or instead of that, we could just simply and elegantly, leave it in the had of the player to decide if he wants to detour for a gun he may or may not want, or let him continue on his merry journey to BR5.