https://www.metv.com/stories/did-you-know-that-the-beverly-hillbillies-became-a-big-hit-in-britain

The Beverly Hillbillies wasn't just for the country folk, it was for the Brits too!

I wonder if the series' popularity in Britain was what prompted the episodes where the Clampetts inherited a castle and spent some time in England?

More than anything, those episodes remind me of the Munster Go Home movie, which I believe came out the previous year. The Munsters also inherited a castle in England.