As far as I know, the "most historic" mill in Scotland is Caerlee in Innerleithen (1788) as opposed to Johnston (1797):
Caerlee (I believe, that name is very recently) is the historic Ballantyne factory, which is now mainly produced in Italy (although a few pieces are still made in Scotland). While Johnstons of Elgin produces 70% under different labels, in the case of Caerlee it is most likely to be 95%.
Just look at this cardigan and compare it to the one in Drake's collection:
For Drake's sake, I hope it's not Caerlee.
if it is, they will need a new supplier because Caerlee just went bust.