Building on green-belt land is not sensible.
It is being proposed in piecemeal fashion so issues like transport and infrastructure are not addressed until the final phase.
In the meantime, local services are put under stress.
Add to that, the envisaged food shortage which led to the EU’s call for farmers land to be used as productively as possible, and you cause even more difficulties. The answer is somewhere between. Croydon is a good example of a transport system adapting to the modern commuter and urban areas expanding in a radial fashion. If the idea is to house key workers, then having them commute from green-belt areas isn’t sensible.