Where any allowance to any person in the service of the Crown is certified F2. . . to represent compensation for the extra cost of having to live outside the United Kingdom in order to perform his duties, that allowance shall not be regarded as income for any income tax purpose.

A certificate under subsection (1) shall be given by the Treasury, the Secretary of State, the Lord Chancellor, the Chancellor of the Exchequer, the Minister of Agriculture, Fisheries and Food, the Minister for the Civil Service, the Lord President of the Council, the Lord Privy Seal, the Attorney General or the Lord Advocate.