Suspect it is mainly the 'red wall' areas.
Brexit was effectively (at least for many) a 'disillusion vote', it was more to do with the 'perceived reality' of many in 'red wall' areas, of feeling left behind, of not being heard. Now after having returned to Labour, on a promise of change and having a massive majority to do so, these areas are once again feeling unheard, left behind; because 'change' has not come as quickly and wide-ranging as they wanted....and was never likely to do!!
So in this election, Reform is picking up on the same 'disillusioned sentiment' delivered in the 'Leave' vote in Brexit.
"Those who do not learn from history are condemned to making the same mistakes"
There are lots of faults (non-representation) in the FPTP system, but with two large parties covering a broad range of the left and the right in politics, a workable format for governance was obtainable, with the odd coalition thrown in.
Now however the 'splits' will be likely to become so massive, that future Governments made up of a variety of political 'stripes', may not be able to 'find their way out of a paper bag', inertia will reign in the corridors of power and decay all around, will increase rapidly.... are there any 'small boats' going the other way?