"how many people could afford to buy the land for a house and build it without being able to live in it for five years?"
Interesting point John B. How about the HCA set aside some plots of this land for self-build on similar terms? Then you'd only need to borrow the money for the build cost (approx £100k) and you could get a further mortgage for the land value upon completion. The result would be some people would house themselves, homes would get built, and those doing a self-build would only pay the cost of the land and the build, not the developer's profit - low cost housing provided with no public subsidy.