Although since the TV series is set in Grassington, they probably would have been drinking Pennine mild.... (whereas "Herriot" actually practised the other side of the A1 in Thirsk)
Ron Pattinson in his Vintage Beer book says that mild (which in the 19th century was always pale) "continued to darken throughout the interwar period, and most examples were either dark amber or brown in color. In some regions, for example the northwest of England, breweries had an ordinary mild that was dark and a best mild that was pale."
I see that Timothy Taylor says that Golden Best is the "last of the true Pennine light milds."
So is Pennine mild basically pale best mild? Or is there something more than that?