Hampstead Heath is actually one of the rare examples of the public successfully fighting private developers to keep land in public ownership[0], culminating in the 1871 Hampstead Heath Act which ensured the land was "always kept uninclosed and unbuilt on"[1].

[0] https://www.heathandhampstead.org.uk/home/origins/

[1] https://www.legislation.gov.uk/ukla/Vict/34-35/77/enacted