Perhaps some of it has to do with class rivalries, which are more acrid in the UK than in the US? In the UK, the wealthy adopted a supraregional standard (Received Pronunciation) through their schooling. Therefore, by retaining one’s regional accent an ordinary person was emphasizing that one was not one of that detested class.
This is a pure guess. But I have heard that features of London’s “Estuary English” are now spreading to other UK cities. That accent is not an upper class one.
Received pronunciation is typically spoken by a very small percentage of Brits- a quick Google search claims around 3%.
There are class differences, but even within classes there are very wide variations- thinking Geordie v Yorkshire vs West country vs scouse vs South London.
I don't know where you live, but as someone who does live in the UK, I think everything really is about class here! It affects every almost interaction, and where contemporary fashion usually masks class, it is one's accent that usually gives it away.
This is a pure guess. But I have heard that features of London’s “Estuary English” are now spreading to other UK cities. That accent is not an upper class one.