One in six living here now we’re not born in Britain
Is that using the entire population? The first thing to do when presented with a statistic is to perform a plausibility check: count all the people you know, and check how many of them weren't born here. Of all the friends, family and people I meet regularly there are four who weren't born here: a German, Pole, South African and my French cousin who is a British citizen through her mother. That would imply a sample size of 24 people. I doubt this is particularly unusual.
Now consider that 21%(one in five; a close number) of the population is under 18 and perform similar calculations.
Also, the birth rate has fallen lower than is necessary to keep the population stable, creating an aging population which in turn leads to a potential finance crisis. This is common throughout the western world.
None of that means we shouldn't control out borders better. Considering that Britain is an island, that ought to be easier than France or Germany which have immigration problems of their own. The only way to
stop the illegal immigration that is the real problem is to make it worse for them here than at home. Anyone who wants to do that to their own country is a oppsing idiot.