The Armed Forces (Aliens) (Amendment) Regulations 2018

Section 340 of the Armed Forces Act 2006 (c.52) prohibits aliens from serving in the regular forces or Her Majesty’s forces raised under the law of a British overseas territory other than Gibraltar. However, the Defence Council may make regulations which exempt aliens who satisfy prescribed conditions from the prohibition.

Link: The Armed Forces (Aliens) (Amendment) Regulations 2018
Source: Legislation .gov.uk