The population density in the United Kingdom is 269 per Km2
I calculate that that means each person has over 3700 square metres. That sounds quite a lot. You can drive through some parts of the UK and not see anyone houses for miles. There is plenty of space but we just don't use it too well, and of course we don't have the infrastructure in terms of hospitals and schools, or even houses to accommodate more people at the moment. With proper planning I'm sure we could accommodate another 20 million in the UK by 2050. If we continue to grow though we will lose our national identity at an even faster rate.
According to More or Less on Radio 4 the world's population could fit on the Isle of Wight, although I think it might put some strain on the schools and hospitals there.