Rather odd that the government has used rural exceptions as the model for the ‘industrial exceptions’ starter homes (20% off) policy.
They have undelivered because if you can get full planning permission why knock down your values? If a site is genuinely underused and redundant you will get permission under the NPPF anyway. My experience with promoting urban exceptions policy in the past was they were only defensible if a site had demand for employment and you could say that the ‘affordable’ housing was the higher priority land use. There will be very few such sites in high demand areas because areas like London, Cambridge, Oxford etc. have big shortages of employment land. This seems a policy designed only for a few midlands towns.
You will pay CIL/S106 on general market housing but they will still be valued more as you wont have the restrictive under 50s first time buyers restriction. So a policy, like rural exceptions, doomed only to deliver a trickle of housing. No excuse for not allocating more land.