My solution is underground cities. Or cities in the sky, I'm not sure which, I havent really thought it through.
"Never a truer word spoken in jest!"
By that I mean, the only way that the UK will solve its population problem is to go upwards and build huge great high-rise flats.
I already live in a road of what were large victorian family houses, which are now mainly flats. There is no room left to build sideways, so I see blocks of flats gradually getting taller.