That is because it's less focused on one city so that demand is a bit better distributed. Zoning law is also more helpful than in London, where laws prevent from building on one of the numerous golf courses in the city.
I have no interest at all in golf, but I'd like London to keep the green space it has.
London just needs to build taller. It happens, but very slowly. Middle class British people don't like anything other than houses, so compared to any German city there are very few 4-6 story apartment blocks in London.
The green belt is not about green spaces. Only 10-20% of the protected spaces are actually parks or forests. Much is unused land or agriculture. No one wants to get rid of Hampstead Heath or Richmond Park.
London just needs to build taller. It happens, but very slowly. Middle class British people don't like anything other than houses, so compared to any German city there are very few 4-6 story apartment blocks in London.