I see this BBC article expands more on what the grey belt is. There are some abandoned areas that are considered green belt so cannot be developed, e.g. an old petrol station…

It’s a tough spot to be in, removing the right to buy. Those who can’t afford to buy a private home will likely have an even lesser chance now. Owning a home gives people an investment rather than simply paying for somewhere to live. I think the biggest issue comes from greed, those wealthy enough to buy multiple homes simply for financial gain.