Two things:-
First, in the new world of COP26 and its aftermath and commitments to reduce emissions it is going to become increasingly difficult to get airport or runway expansion through the planning process, certainly not with any speed and as it is the planning system in UK is so unwieldly and protracted without adding the environmental considerations.
Second, there are already too many airports in the UK. Do we really need BHX and EMA, MAN and LPL, GLA and PIK, BFS and BHD or NCL and MME? As PUG says whatever the proponents of LPL might say, given decent surface connections LPL's "local" airport is essentially MAN. You could of course argue, with a a good deal of justification, that the surface links aren't that great at the moment between any of the airport pairs I've listed.
I think its pretty certain that in BHX, for example, were applying for their runway extension today, as opposed to half a decade ago they would have little or no chance of getting approval for it. The world is changing, whether for better or worse is debatable, but it will impact on air travel (outside of the sacred London of course!).