I don't blame Brexit as the driver for this WM. I don't even say that Brexit was the final catalyst which drove the situation over the edge. It was a combination of Brexit, Covid and the natural tensions created by the regional governments/assemblies.
Before the regional devolvement, people in Scotland, Wales and Ireland only had Westminster to raise their issues in. It is fairly easy to block out Scotland at 10% of the UK, let alone Wales and NI.
In FIFE the miners strike had an absolutely devastating effect. The mines flooded, none of the miners went back to work. Whilst England just went back to the mines and started working again, FIFE went 50% on the dole and the economy was still transitioning and not up to absorbing so many unemployed. Even today FIFE has the second highest drug usage in Scotland and the third highest in Europe.
FIFE was by no means the only part of Scotland, or the UK, which paid the price for the excesses of the Unions in the 70's and early 80's, but it was a glaring example. FIFE is heavily rural, NE FIFE was a Tory seat for a long time with Sir John Gilmour, but nobody would vote Tory there now.
It's like a tree which is rotten to the core and suddenly struck by lightning, it explodes.