not sure why people are so angry at OP - I think a lot of people don't know what a tiny tiny proportion of land in this country is actually built on.
Even these comments about 'oh yeah let's just build over the whole countryside,' suggest people don't really understand what OP is saying - it's not like we have built on 80% of land and we now need to stand firm to keep the last few remaining fields green.
This article www.bbc.co.uk/news/uk-18623096 which states that " the figures suggest Britain's mental picture of its landscape is far removed from the reality:"
"The proportion of England's landscape which is built on is 2.27% [...] According to the most detailed analysis ever conducted, almost 98% of England is, in their word, natural. Elsewhere in the UK, the figure rises to more than 99%. It is clear that only a small fraction of Britain has been concreted over."
You don't need to look at some daily mail pictures of green fields to see this - just do satellite on google maps.
Of course this doesn't mean all this land is suitable to be built on. But when we are talking about overcrowding and lack of infrastructure we shouldn't be put off by worries about lack of land and NIMBYism. We were perfectly capable of planning and building new towns from scratch post war, why can't we do so now? We have land, we have people wanting to work in new hospitals, schools, etc. (does it matter if they were born here or not?), and a population desperate to access them. We are in a better position from 70 years ago as new technology means that there is no real reason for the vast majority of government hubs or large buildings to be based in capitals other than the prestige factor and traditional thinking.
It just needs someone with courage in the government to commit to large scale planning, and obtain funds from wherever needed, even if it's an unpopular move like raising taxation.