The thing that struck me as well is the close association with football - as noted many times, of course, by Robert Elms in The Way We Wore.  I certainly recall visiting Arsenal in 1970 when the whole purpose of going was not to watch the match but to pose in your new sheepie and Royals!

Russell / Brideshead what are Royals..? I remember my elder brother talking about them when we were younger but I've never to my knowledge seen a pair....

^ Is this a case of a brand name being adopted as a general term? (i.e. "Hoover" instead of "vacuum"?)

Back in 1970 'Royals' were called by that very name in The Ivy Shop.  That was what we would now call the 'brand'.  As Russell says they came in a longwing style (see link above to 'Royal Brogue') and smooth or 'plains' as we tended to call them.  A Norwegian fronted style became available a little later and was called a 'Gibson' but I cannot recall if the brand name was still 'Royals' - I think it probably was.  R_S may know......