There is a big difference between land available for building and how much of that land is actually owned by the developers and councils who want to do the building. Land is expensive has been for decades, with all the developers fighting for this land if we increase house building, prices will rocket.
While overall there may be more than enough land for building, is it in the places the housing is needed. A big brownfield site in parts of the north west, where house prices are minimal anyway and many of the cheapest cannot find a buyer is really not much use.
Large brown field sites are often the result of major industries closing down and that means the land is contaminated from decades, if not centuries of industrial waste and remediation, ground consolidation can be very expensive, which adds further costs to the houses.
Developers usually work on holding a 5 year landbank. They can be told to stop hoarding that land and build on it, assuming buyers can afford what they build, but what then? Hundreds of developers, with no land to build on all competing for what land is available, and driving up the price of land further.