Adult Social Care Glossary, v 0.6

A person is subject to immigration control if s/he requires leave to enter or remain in the UK. Every person requires leave to enter or remain in the UK unless s/he is:

a British citizen
an Irish citizen
a Commonwealth citizen with the 'right of abode'; or
a citizen of an EEA member state who is exercising one of his/her freedom of movement rights which arises from the UK's membership of the European Union (or during the transition period once the UK leaves).

All the above (British, Irish etc.) have a 'right to reside' in the UK. Any person who requires leave to be in the UK but does not have it is in the UK unlawfully – unless s/he has been granted temporary admission.