It’s remarkable how easily we can slip into the assumption that what currently is, has always been so. Extended 30ºC heat in Britain. The most well-educated people voting for leftwing parties. Japan and Italy facing demographic crises due to old and ageing populations.

So it may come as a surprise that in 1985, Britain was considerably greyer than Italy and Japan. The UK had the second oldest population in Europe, with 15 per cent of people aged 65 or above, compared with 13 per cent in Italy, and just 10 in Japan.

But fast-forward 37 years. The UK is now the fourth youngest country in Europe. It’s gone from an elderly population roughly two percentage points larger than the EU average, to two points smaller.

What is the rejuvenating elixir? Immigration, for the most part. According to my calculations, had the UK kept its borders closed for all this time, 22 per cent of the population would now be pensioners, rather than 19 per cent. An already threadbare workforce would be one-quarter smaller, and a crumbling health and social care system would be absent a fifth of its most vital employees.

Instead, a steady influx of working-age people, and the fact that immigrant communities tend to have higher birth rates than the native population for several generations, have kept Britain’s population remarkably dynamic over 40 years in which peer countries have grown top-heavy.

Yet too few of us realise how remarkably lucky Britain is in this regard, nor how fragile our grip on this privilege may prove to be.

Fifteen years ago, the US polling firm Gallup began a series of global surveys, asking people whether they would like to move permanently to another country. The first three editions of the poll placed the UK second only to the US as the destination of choice. But in the fourth, done in the heat of a Brexit campaign riddled with anti-immigration rhetoric, the UK slipped behind Canada and Germany to fourth, and in the most recent, it ranked seventh.

While Britain is losing its sheen for a new generation of entrepreneurs, inventors, scientists and doctors, the competition to attract them is growing more fierce.

Declining fertility rates have gone from primarily a first world problem to global as social and economic development has proliferated. Only one country — Chad — has yet to see its birth rate fall. As a result, more and more rich countries have ever greater need of migrants, and the pool available will not keep growing forever.

All this makes it self-sabotaging for the duo vying to be UK prime minister to seek to outdo one another in hostility towards potential migrants.

For now, Britain enjoys a relatively high immigration level, with new arrivals helping keep a listing country afloat. But it has flattened out and it would be naive to think such a boon will not be dented by diminishing economic incentives and increasing belligerence from the country’s leaders. If you’re going to indulge in vice signalling, be careful who’s listening.

john.burn-murdoch@ft.com, @jburnmurdoch