You might find it useful to look at where Green Belt lies in England. It is a buffer to prevent coalescense and only applies to England. Capital G and capital B is normally used as it has a legal definition.
https://urbanistarchitecture.co.uk/resources/green-belt-map/
Green Belt is not 'green land' which is a loose term to describe something that is neither brownfield nor Green Belt and nor protected land such as National Parks. Most green land is farm land and because land is used in some way you will find farmland adjacent to all other land unless it is unusable such as marshland. A golf course is brownfield incidentally and so is disused quarry that could be a nature reserve one day.
Broadly, when we talk about green land we are talking about building on farmland. Going back to my earlier post, is it better for us as a nation to (1) build on Green Belt (2) build new towns in the middle of nowhere or (3) build on those green fields adjacent to existing settlements (ie non-Green Belt). My view is (3) is the better option for a variety of reasons but mainly the loss of our natural world and the loss of the perception of tranquility fundamentally changes our places. We still have some tranquil areas as opposed to many built up countries such as the Netherlands.
Note that building out 10% of half our settlements is equal to the number of new homes that Labour will pledge. With no new towns. And if we look at only half of these then we can ensure those Green Belt settlements are in protection.
@enchantedsquirrelwood raises some excellent points. I will come back to this thread a bit later.
There are though many brownfield sites that could be used for housing. The majority of new out of retail parks built in this Century are half empty. These are not traditional High Streets, so they are a failure of planning and retail to keep track of each other. There is no joined up thinking anywhere.
In fact, they have built enormous sheds on green land the other side of the retail parks to store goods for Amazon because people want home delivery. Who gave them consent for that when they could have repurposed the existing almost redundant retail parks? We know it is 'easier' to build on green land because it is cheap and has no recycling costs.
There is much for Labour to get to grips with here. It will be a challenge and one they can rise up to, but they need to be aware they are dealing with a few very rich and powerful people in the property sector who play a very hard game when it comes to controlling land in England. Most of these people run private family companies out of the public domain and have more money than they could ever need. These are the people lobbying the Conservatives to abolish Inheritance Tax. These make up the majority of the 4% who rightly pay it. I know who they are.