Not really happy days. A decent democracy needs a decent opposition. I see that the new gov are planning building not just on the grey belt but even on the best parts of the green belt. Any owner who refuses to sell will be subject to a special order taking their property for below market value. Environmentally this is a disastrous policy but there will be nobody in the commons to oppose it properly.
Given it's the LAs that are deciding where to build in their own areas, they're unlikely to choose the "best parts". Best for building, and near to existing infrastructure, but most aren't daft enough to pick the most beautiful or most used green spaces, and concrete them over.
The 'market value' thing has been an issue going back years. It's not screwing over landowners. There are companies which look at where they think building might take place in the future, and then they buy the land at a low price, because currently it can't be built on. If the land is eventually reallocated for building, then they have been able to sell it for a huge profit. Under the new rules, they will still get a profit, but nearer to the one that they would expect to get based on the land being green belt.
They're not taking a risk, because the land they buy is cheap, and if it doesn't get rezoned, they still own it, and can sell it on. They have essentially done nothing for society, except make house building, house prices, and the cost of the related schools, roads and other infrastructure, more expensive.