Exactly. And yet "why can't they build on brownfield?" is the immediate cry whenever anyone even starts looking sideways at the green belt. Nobody is terrified of brownfield, which is why I strongly suspect that brownfield alone isn't going to make a damn bit of difference; to steal a phrase, "if building on brownfield changed anything they'd make it illegal".
> it's painfully clear to me that high house prices was a deliberate choice
Yes, very obviously. And not just in the obvious "pandering to donor builders and smug homeowners" sense, either; debt creation through mortgage lending is at the heart of UK monetary policy now. Probably the most extreme example I've ever seen of jam today at the cost of long-term disaster.