"Of course present levels aren't sustainable"
Tug that is the first time you have even hinted at that.
Your previous;

the 'our overcrowded little island' argument is bankrupt at every level.

Our island is still not overrcrowded

immigration population who have a younger age profile and a higher fertility rate.
But so what.

Now we agree, so that is fine. End of our discussion.
What should be done? Almost all other countries apply limits to immigration.US style quota system.Australian points based system that, unlike ours, actually limits the numbers entering.