The figures are from the ONS mid-year population estimates, for regions and local authorities. A number of data sources are used for the estimates: The General Register Office, The International Passenger Survey, The Higher Education Statistics Agency, and The National Health Service Central Register. The data accounts from long-term migration trends (people staying for more than 12 months) and estimates are made for different components of change. More information can be found here.
National Insurance Number (NINo) Registrations
These statistics are a 100 per cent extract of the volume of adult non-UK nationals registering within the UK for a National Insurance Number, which they need to work or claim benefits / tax credits. Figures are based on when the person registers on the HRMC Recording and PAYE system, which may be some time after they entered the UK. These statistics are not a direct measure of long-term inward migration, and have ‘national statistics’, not ‘official statistics’ status.