Excellent point. There is a surplus of housing in many regions, particularly run down, ex industrial towns, ex-seaside towns, where there is street upon street of empty/derelict properties, lots of derelict industrial buildings, etc.
Trouble is the developers aren't interested because the demand isn't there, due to low local population, unskilled locals, high unemployment rates, few decent employers, etc. They're not going to go to the cost of demolition, costs of dealing with contaminated land, etc., only to build very low cost housing that may well not get sold at a high enough price.
Developers concentrate on the "attractive" towns and cities where there's high demand, so high prices!
The London-Centric nature of employment, due to the obsession with a service industry relying on financial services rather than actually making stuff, has caused a "brain drain" of people having to move to London (and a handful of other big cities) after University to get the decent jobs, leaving behind a vacuum in their home towns where the only jobs are NMW retail, hospitality, care work, etc.
As an alternative to "build more homes", perhaps the government should reverse the brain drain into London (and a handful of other big cities), and encourage employment into the regions, encourage manufacturing firms to start up in the regions, etc. So as to spread our population more evenly over the entire country instead of concentrated into small pockets. After all, the infrastructure in the regions, smaller cities, towns, etc is already in place and often under-used, and could easily cope with more people living in those areas. After all, there are schools, GP surgeries, etc closing down in run down towns because they have too few pupils/patients, likewise bus routes etc being scrapped because of too few passengers.
So how about it? How about spreading people out more across the country? Far cheaper and more sustainable than just lazily allowing the existing few big cities to spread more and more, putting ever more strain on the infrastructure?