Roy Friendship-Taylor (The Crowded Isle - 21 October 2013) suggests that the number of people registered for National Insurance shows that the current UK population significantly exceeds the official ONS estimate of 63.7 million. He can be assured that it is possible to have an active National Insurance number without being resident here. There are , for example, over a million UK pensioners living abroad. The ONS estimate is based on internationally agreed definitions of population and should be trusted.
Director of Population and Demography, Office for National Statistics