You are correct that the units tended to wear one type of headress or the other and the orders for headress were generally divisional level.
That doesnt mean that different units didnt use different headress. Corp units would be on the same battlefield with different head gear and religious units could use different headgear too
My 14th army have hats for one unit and the helmet for my west Africans. I also have sikhs in turbans.
They are mainly peter pigs with some PP heads on some individual battlefront Italy figures to give some variety.
I found that it was necessary to do some headswaps for Piat teams, Flamethrowers and mortar teams and artillary crews.
Peter pig do carriers with the heads seperate so they are the easiest to do conversions.
I did not use any eureka because of the gaiters although It would be possible to use their bardia Australians in greatcoats.
Most of my vehicle crews were battlefront & command decision figures.
The battlefront italian theatre British troops are good for helmeted troops.
Paratroops in berets are also useful.
In burma, the british used US flamethrowers so the PP figure is the only figure with the right equipment
My own 14th army uses PP, CD, Battlefront and QRF to get the right "stuff".
