INTM153290 - Description of double taxation agreements: Diplomats
This Article ensures that the taxation privileges to which a diplomat etc. is entitled under general international law or under specific domestic legislation of each country, for example ITEPA03/S300, are not overridden by the provisions of a double taxation agreement.
Some agreements also contain a further provision that diplomats etc. of one country serving in the other country are not deemed to be residents of that other country if they are subject to tax there only on income from sources in that other country.
INTM154060 gives information about what to do if you are asked to comment about the residence status of a diplomat etc.