With the way ITV was 15 separate companies years ago I have heard that a number of ITV stations showed certain US comedies that weren't networked. I have heard of a comedy called Gilligan's Island (that wasn't meant to be the best US sitcom) and I am not sure if this was shown in the UK. I live in the ITV Central region (formerly ATV, of course) and apparently Mister Ed was never shown on ATV.
I am pretty sure that by the 80s most US shows were networked over here, such as Diff'rent Strokes and Family Ties. But back in the 60s and 70s I am pretty sure there were quite a few US sitcoms that weren't networked. I think The Beverly Hillbillies was, but not Petticoat Junction.