On the Daily Politics a while back Claire Fox from the Institute of Ideas (who you'd think should know better) attempted to justify the liberalisation of planning laws by saying that only 10% of land in England is developed. A New Statesman leader said this back in March this year:
‘Only 10 per cent of England (and 6 per cent of Britain) is developed... The UK is 60 million acres in size, of which 41 million are designated "agricultural" land, 15 million are "natural wastage" (forests, rivers, mountains and so on) and owned by institutions such as the Forestry Commission and the Ministry of Defence, and four million are the "urban plot", the densely congested land on which most of the 62 million people of these islands live...’ http://www.newstatesman.com/society/2012/04/land-government-million
In terms of whether to build on green land or not crude land area is not really the way to consider this issue. Look at these figures: average biologically productive area per person globally was approx 1.8 global hectares (gha) per capita in 2006. Average ecological footprint in the UK is 5.45 global hectares per capita (gha) (see http://en.wikipedia.org/wiki/Ecological_footprint). This means that not only have we used up all the available biologically productive land in the UK we are actually drawing greatly on large amounts of land from abroad as well as allowing carbon levels to build up in the atmosphere because there is insufficient productive land and water to absorb it fast enough. Our 5.45 gha/person ecological footprint is three times greater than the average productive land per person available worldwide.