EIM43020 | Residence or employment in the United Kingdom: government or local authority remuneration or pensions
From HM Revenue & Customs · Employment Income Manual
Remuneration or pensions paid out of public funds in respect of services rendered to the government or a local authority in the discharge of functions of a governmental nature, will be taxable only in the State making the payment unless the individual:
is a national of the other State and
is not a national of the State making the payment.