Which British Lord has not been Secretary General of NATO?

Lord Lockhart has not been Secretary General of NATO.

The Secretary General of NATO is an international diplomat who serves as the chief civil servant of the North Atlantic Treaty Organization (NATO). The Secretary General is responsible for coordinating the workings of the alliance, leading NATO's international staff, chairing the meetings of the North Atlantic Council and most major committees of the alliance, with the notable exception of the NATO Military Committee, and acting as NATO's spokesperson.

Lord Ismay was the first Secretary General during 4 April 1952 – 16 May 1957, Lord Carrington was the sixth Secretary General during 25 June 1984 – 1 July 1988 and Lord Robertson was the tenth during 14 October 1999 – 17 December 2003.