The Aneurin Bevan board, which has the most cases in Wales, has categically stated that the main reason they have so many is because they tested so many people. They had an early case in a health worker so tested loads of nhs staff.
They've also speculated that another reason is the number of commuters to London from there.
The holiday home areas have way fewer confirmed cases. I mean, some people might have a holiday home in Monmouthshire but that health board mostly covers newport and the south east valleys (shiteholes).