The debate keeps veering away from population and back to allocation of resources. This isn't the point. Yes we could house and feed far more people but not under the current capitalist system we currently have. It's not good for anyone if more and more people are added to the desirable, yet wholly unsustainable way of life that is modern day western civilisation. Whether or not it's morally right that the UK takes more migrants (and I stress migrants NOT refugees as of course we do have an obligation to take the latter) is besides the point. We can't if we want to create the sustainable, self sufficient country we need to sustain life.
We've run out of land. And yes we could ban second home ownership and ensure all UK housing is actually lived in, but that still doesn't solve the issue. No amount of building affordable homes is going to solve this, quite the opposite, it'll add to it because the only way out is to start respecting the planet and living with nature not against it.
We need to free up quality agricultural land for organic farming and maintain wild green spaces for absorbing carbon emissions, reducing global warming and to provide civilians with the natural environment they need for their health and well being, including fresh air and water, which at the moment we don't have, or have in dwindling supply.
Changing the way we live just so we can cram more people in because it's perceived to be the right thing to do doesn't solve the problem. All that will happen is that the native population and the immigrant population will go down with a sinking ship. Whereas much of the immigrant population will presumably have another home to go to and dual nationality to take advantage of, much of the native population won't, so that's one privilege the natives are missing out on; the privilege of abandoning the sinking ship when the time comes.
Sure, we need to take away from the people who have too much,
The gross and disproportionate amount of wealth distribution is a travesty in the UK. But we also need to reduce the population, give back the land, go back to a simpler, more holistic way of living that will involve a great deal of doing without, and leave it there.
Then we can look at how to incorporate more migrants into this new, sustainable environment under a fair, sustainable and ethical process that benefits all. It's the only way out.