Why don’t we know how many people are in Britain?

How many people live in Britain? You would think there would be a straightforward answer, but it eludes some of the nation’s brightest statistical minds.

The problem of undercounting has worsened in recent years, largely because of high post-Brexit migration

This week the Office for National Statistics (ONS) projected that our population will grow by some 4.9 million people over the next seven years, bringing Britain’s official population to over 70 million. The bulk of that population growth will come from immigration – nearly ten million people.

But can these projections be trusted? Never mind how many people will live in Britain in seven years, we do not know how many people are living here now. This is despite the fact that – going by the number of CCTV cameras per (estimated) capita – Britons are among the most surveilled people in the world. Big Brother may be watching, but he isn’t counting.

While the ONS does produce mid-year population estimates, these are often considerably wide of the mark. The last national census, in the spring of 2021, proved that point. The total reported population was off by nearly 300,000 from the ONS’s estimate for the year.

Other estimates fare even worse. The Annual Population Survey, which samples about 150,000 households nationwide, has seen response rates plummet from around half in 2017 to less than one in five last year. The APS is so useless that its national statistical accreditation was suspended in October.

Unlike some other countries, such as Sweden, Britain has no centralised population register. We don’t count people in or out of the country, so our population figures are based on rough estimates. Imposing such checks at the border is more difficult than it sounds. Dual passport holders and the UK-Ireland Common Travel Area both present logistical hurdles.