> For Brexit, the government claimed there were 3.7 million people from other EU countries living in the UK and the actual number who came forward to apply for residence was 2 million more than that.
Source seems to be this https://blog.ons.gov.uk/2021/07/02/are-there-really-6m-eu-ci... , and it notes quite clearly that "EUSS data should not be used as an indicator for how many EU nationals are living here. "