According to the CPRE,
there has been a significant loss of Green Belt since 1997 and more is under threat from development. Since 1997 over 1,100 hectares of Green Belt have been lost each year and at least 45,240 homes have been built on Green Belt land. This is of course related to immigration. Immigration = more people = more housing needed, and that new housing can't all go on brown land.
So how do we save our countryside? Well I suppose we could vote BNP.
You can see the full report here: CPRE - Campaigns