The Campaign to Protect Rural England (C.P.R.E.) has an up date on brownfield sites to mark a 'change of directive' to local government.
Link to posting here.
Two points come to mind and I shall make them briefly.
Tyneside has numerous undeveloped brownfield sites. They are under developed because private out-for-the-biggest-profit builders have stealthily or not so stealthily bought farmland in the Green Belt to cash in on the 'aspirational country living' market and wouldn't buil;d on brownfield unless forced to. Fortunately most are gerry builders who are being increasingly found out. Answer? Get Danish, Dutch, German or Swedish builders in to the do the job the tax evading British sprawl creators won't do.
Second point is that much of the Green Belt as it stands is merely rye grass and barbed wire. That to needs creative thinking. But is not getting any.