Many local authorities who have large numbers of east European Migrants have long suspected this, with NHS and School rolls records widely at variance with mid year population estimates suggesting massive census under enumeration.
The applications for settled status has confirmed this. Around 1.1 million may have left Britain after Brexit, leave supermarket shelves empty, and fruit rotting in fields, but the number of working age was underestimated by around 1.4 million, suggesting at least 2 million underestimated total population. England’s demographic baseline for use the standard method will have to be resent. We will soon have a new census, but now live data from GP rolls collected as part of COVID are now far more accurate, why not use those instead?