Surely it's the other way round? Only 11% of land is built up, but that is in the most desirable / liveable/ workable areas, so most of the people that live there feel much more overcrowded than they really are, because although only 11% of the UK is built on, that 11% is where they live. Therefore we should be thinking hard about whether it's a good idea to build more in those areas, because people there are already feeling suffocated.
I also immediately wondered how much of the remaining land is actually buildable upon, and is not mountain, flood plain, nature reserve, bog or moorland.
"The houses are dead and there is too little that grows and is glad." :-(