St George of England : ONS reported we have over 500,000 illegal immigrants here this year in the UK.
ONS reports that the 2021 census showed there were 545,000 "usual residents" in England & Wales who had an address outside the UK one year before the census. A "usual resident" is one who is not a temporary resident, like a student or visitor on a limited visa. As kramer points out, this is the total number of immigrants and is no indication of illegal immigration.