Hi there, sorry to be a bother, but I think late-game heavy infantry should use the Italian model and mid-game heavy infantry should use the default English ones (English only appears around 1100 or so anyways, great helms wouldn't be completely rare by then), as even early in the hundred years war, the English and French were fighting in plate armour

UPDATE: I've added some early unit designs to serve as the unique early game units for the Frisians. At present, they are the only culture that uses the englishgfx graphical culture for their units in the early game and, since English cannot appear before 1100, they are the only culture that will ever be able to use these early game units. Hope they're to everyone's liking!

Unfortunately I cannot seem to replicate this issue on my end. The problem you are experiencing was present in some of the other, older mods (namely the Norman and Anglo-Saxon ones) but they have since been fixed in a patch and it shouldn't be present in this mod which was released with the patch already implemented. Are you using any other mods, not made by me, that alter unit models? It can result in a conflict, even if they don't necessarily modify English units specifically.

Also, I noticed that you've posted this here and in the Manic's Units mod comments. Which one of these mods are you using? This may help narrow down the issue.

So there's a bug where the helmets of the late English troops, both the cavalry and heavy infantry varients, get merged with their mid game counterparts as both models obviously try and share the same space.