The Earth has been calculated to be able to sustain a maximum population of 10 Billion. We are at over 7 Billion now. At the present rate of population increase the 10 Billion limit may be reached with in 30 years. We will be lucky if the Earth as the whole planet is still habitable in 30 years never mind the UK !
http://www.livescience.com/16493-people-planet-earth-support.html
Even the 10 billion limit is only reachable if everyone on the planet is prepared to live at the standard of living that Europe had in the 1950s!
If every one on Earth wanted to live at the standard of living that the USA has today we could only support 2.5 Billion people.
This is NOT about immigration it is about the entire worlds population.
The first thing to go will be water, we are using fresh water at over 3.5 times the level it can be replaced. California in particular and much of the rest of the South of the USA is already in a sever drought
It takes 3.5 times as much land to feed people as with meat as it does to feed them with vegetables and grain, so ALL animals will have to be killed to make way for grain to be grown on the pastures they feed on.
No politician is prepared to even admit that this problem exists, let alone try to do something about it!