Agreement between UK and Thailand
UK-Thailand Double Taxation Convention signed 18 February 1981 (publishing.service.gov.uk)
2) (a) Any pension paid by the Contracting State or a political subdivision or a local authority thereof to any individual in respect of services of a governmental nature rendered to that State or subdivision or local authority thereof shall be taxable only in that State. (b) However, such pension shall be taxable only in the other contracting State if the recipient is a national of and a resident of that State.