As far as I am aware George Monbiot is not a PFANS supporter.
It would be sad to see PGC closing but hardly the end of the world. Look at the upsides: The area of Portobello Park that is being used to house the school could be replaced and things that "once gone are gone forever" would make a staggering comeback- whoosh the Park is back. PPAG would be dancing in the streets. ok they don't have enough for an eightsome reel but they could muster a Grand Old Duke of York with a minimalist March.
Suddeny around 50 acres of "inaccesible" open space would become accessible, our open space stats would go through the roof.
And there are other options; Rugby Pitches, Tennis Courts, Allotments, Skateboard Park and so on.