Permanent Under-Secretary of State for Foreign Affairs

The Permanent Under Secretary of Foreign, Commonwealth and Development Affairs, sometimes known as the Permanent Secretary of the Foreign Office, is the most senior civil servant in the FCDO. A former PUS, Sir William Strang, described the role as being "the Secretary of State's principal adviser on policy and … his principal agent for administration". And the UK government website, in reference to one of the former Permanent Under-Secretaries, Sir Simon Fraser, agrees calling him "the senior policy advisor to the Foreign Secretary".

Source: Wikipedia — Permanent Under-Secretary of State for Foreign Affairs (CC BY-SA 4.0)