around 1975. "Ring" doughnuts didn't get to the UK until about 10 years after that...

erm, hate to tell you this my fishy friend, but I ate ring doughnuts from around 1967/8 in Maidstone, Kent -

... and I remember serving them (and eating them!) in my Dad's transport café in the late 1950s.
They were certainly commonplace in London by then - and I think they were popularised by American Servicemen either during or just after the 2nd World War.