the council could easliy add a condition on any large development planning permission (section 106 IIRC) that requires that an appropriate local facility is created, it's just they rather squeeze the developer to subsidise a bus route, road widening etc. Brownfield sites can be remediated in most cases and it is better done properly rather than by a bunch of amateurs doing it piecemeal. In most cases I imagine it would be a "scrape and replace"
councils are also under a lot of pressure to identify land for housing devlopments which goes into their planning policy. GB's obsession with the need to build "affordable" homes for the rapidly increasing population in England means they can't afford to "lose" sites to allotments in urban areas. The reality is that house building has collasped and will only start again very slowly. Still doesn't get your allotment though.