[bobthepanda]

There was a policy in the early part of the 20th century to establish a green belt of preserved space around London.

This didn’t stop people from sprawling ever further, they just skipped over the green belt.

[Symbiote]

Leaving the gap was also policy. Stevenage was the first: https://en.wikipedia.org/wiki/Stevenage#Stevenage_New_Town