I think that British cyclists were quite eclectic in their choice of components in the 60s, so I wouldn't sweat having a "group" at all. Britain's economy was almost decimated by WWII, and didn't recover that quickly. British people cycled not only because they had a pretty strong tradition of it, but also because not that many of them could afford automobiles and their upkeep. And frugality (bang for the pound) was therefore a sort of tradition, even in club cycles and higher-end examples; seems to me anyway.