Exhibition about the Northern line extension that wasn’t built

Had things turned out differently, the Northern line would be a lot more northern than it is, and a new exhibition has opened to explore that planned but never built extension.
What is known as the Northern Heights plans would have linked up several branches of the Northern line, and while some were killed off by WW2, one part was killed off by the arrival of the Green Belt around London, and it was that which killed off the extension this exhibition is looking at.Although construction had start