Steve, I was aware that the vast majority of plate was home produced, and certainly all the later production. I'm not altogether sure why manufacture was continued in metric sizes, probably just because that's always how it had been!
I actually just measured a couple of Cromwell turrets to confirm this and found the following.
Cromwell turrets used a 1/2" ms inner shell with a 20mm BP roof welded in and 1 3/4", 2" and 2 1/2" BP plates bolted on. So not all armour was metric, perhaps it was just the thinner sizes. Certainly Cromwell engine decks are 14mm and Churchill VII turret roof is, like Cromwell, 20mm.
All grist to the mill!