This inspired some amusing historical research.
I'm thinking that, after WWII, BSA wanted to design-in bearings for which they could choose more sources than, I assume, just SKF. The rest of Europe, except Ireland, had already completed, or was close to completing, metrication before the war. If they were going to choose a standard for something that they couldn't make themselves, it made sense to choose what most of the rest of the industrialized world were using. (Emphasis on "me thinks".)
Richard L.