the population is currently 64m, and is expected to grow to 72m in 15 years, and to 81m by 2060. is this sustainable in terms of pressure on resources and quality of life? nobody knows. all that can be said is that there's an increasing number of aged population to support and because the UK social service is one huge ponzie, it requires increasing numbers of new entrants to support it. if population numbers were reduced (by whatever means), it will result in budget cuts which will make Northamptonshire's difficulties look like a temporary hiccup, or tax rises so big that even Jeremy Corbyn hasn't thought of them yet.
against that, where is the growing population going to live - is it environmentally acceptable to devote ever more green land to housing? there needs to be a debate, immigration may well be part of that. more people, more food, more houses, for ever? nope, unsustainable.