United Kingdom national (less formally, British national) is a term used differently in various United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 Acts of Parliament

Act of Parliament

An Act of Parliament is a statute enacted as primary legislation by a national or sub-national parliament. In the Republic of Ireland the term Act of the Oireachtas is used, and in the United States the term Act of Congress is used.In Commonwealth countries, the term is used both in a narrow...

. Most commonly, it refers to the following classes of persons:

• British Citizens;
• British Nationals (Overseas)
  British National (Overseas)
  British National , commonly known as BN, is one of the major classes of British nationality under British nationality law. Holders of this nationality are British nationals and Commonwealth citizens, but not British Citizens...
  
  ;
• British Overseas Citizen
  British Overseas citizen
  In British nationality law, the status of British Overseas citizen is one of several categories of British national. A British Overseas citizen does not have an automatic right to live in the United Kingdom.-British Nationality Act 1981:...
  
  s;
• British Overseas Territories Citizen
  British Overseas Territories citizen
  The status of British Overseas Territories citizen relates to persons holding British nationality by virtue of a connection with a British Overseas Territory.-British Nationality Act 1981:...
  
  s;
• British Protected Person
  British protected person
  A British protected person is a member of class of certain persons under the British Nationality Act 1981 associated with former protected states, protectorates, mandated and trust territories under British control...
  
  s; or
• British Subject
  British subject
  In British nationality law, the term British subject has at different times had different meanings. The current definition of the term British subject is contained in the British Nationality Act 1981.- Prior to 1949 :...
  
  s (as defined in the British Nationality Act 1981
  British Nationality Act 1981
  The British Nationality Act 1981 was an Act of Parliament passed by the British Parliament concerning British nationality. It has been the basis of British nationality law since 1 January 1983.-History:...
  
  )

In some other Acts, e.g. the Atomic Energy Act 1989, the term "United Kingdom national" does not include British subjects and British protected persons.

Except for British Protected Persons, all the above are also Commonwealth citizen

Commonwealth citizen

A Commonwealth citizen, which replaces the former category of British subject, is generally a person who is a national of any country within the Commonwealth of Nations....

s. British Protected Persons, like Irish citizens, are in a unique category in that they are neither Commonwealth citizens (British subjects in the pre-1981 definition) nor aliens.