I think they have always have been like that, its for when the cab is in the middle of the formation.
Northerns are the same.
Not quite - It seems to have only very recently come down to the Great Western. If you look at the Local Lines units, it has the door controls but as you'll know the door is as part of the livery. 150202/216 / 001/002 arrived with their cab doors painted in plain Blue too. Only since the very recent repaints / DDA modded units have come back, the cab vestuble doors have been painted Pink or Silver as per this. Incidentally, Central Trains did the same with their 150/2 vehicles as well, probably as a result of the /2s being switched in and out of their faux /0 series units.