At least 1.5 million houses are second homes. Fill these up permanently and there would be no need for extra house building in Forest Hill.
We have a situation where people living in the largest houses have multiple properties while the majority of the population are expected to live in smaller and smaller spaces.
If there was some disincentive for the richest people to own second homes then house prices would slowly become more affordable, although I suspect it would actually be bad for the economy as they would just buy their second homes in France or Spain and take their cash there for a few months of the year.