Growth, growth, growth
But how? The UK has a labour shortage. From farm labourers and fruit pickers, to doctors and nurses. And a labour shortage has a very negative effect on economic growth.
One answer is, of course, more babies. Currently, we're seeing 1.6 babies per woman, and a steady population needs about 2.1. And it's hard to see how that 1.6 could be increased. Tax incentives, maybe? Awarding a medal "Hero-mother of the United Kingdom" if you have four or more babies? But I can't see any such scheme having a significant impact on the decision whether to have more children. Also, there's a big lag - even if we started heavy swiving tomorrow, the babies wouldn't reach the workforce for 19 years or more. And during those 19 years, they would be a burden on our housing, our health systems, our schools and so on.
And governments don't look that far ahead anyway, so not only it wouldn't work, also it won't be tried.
So what's the alternative?
My grandparents came from Russia, about 120 years ago. They were immigrants, refugees from the pogroms. Russia didn't want them, but the UK welcomed them (welcomed isn't quite the word). Immediately, my grandparents (and later their children, grandchildren and so on) joined the labour force.
But we've reversed all that. We left the EU to "take back control", in particular of immigration. That didn't work, of course, because much of UK immigration was from outside the EU.
But maybe we could encourage more immigration from European countries like France, Germany, Italy, Spain, Poland and so on? And how could we do that?
Leaving the EU wasn't a good way to encourage immigration from Europe.