Is it right that more and more greenbelt land in the UK is being built upon for new houses?

There is a little bit of a housing crisis in the UK where there are not enough homes for the growing population. The elderly are living longer, and so there are more and more citizens in the country each year. Understandbly something needs to be done to provide enough housing for the public in the coming years. But building homes on greenbelt land should be the last resort, and I just don't believe there is a viable argument to build on greenbelt land as of yet. My main cause of argument here, is that there is an incredible amount of derelict buildings which should be tranfsormed into new homes first. Derelict buildings are not only a waste of usable land in towns and cities, but also degrade an area by becoming a target for graffiti and litter. Before we start building on protected greenbelt land, unprotected and should be built upon first.