According to Wikipedia (not always the most reliable source, admittedly) Lancaster was the first
British pilot to use a Martin-Baker in an emergency - from a prototype Armstrong Whitworth A.W.52 flying wing.
https://en.wikipedia.org/wiki/Armstr...itworth_A.W.52
On 30 May 1949, while diving the first prototype at 320 mph (515 km/h), test pilot J.O. Lancaster encountered a pitch oscillation believed to be caused by elevon flutter. Starting at two cycles per second, it rapidly increased to incapacitating levels. With structural failure seemingly imminent, Lancaster ejected from the aircraft using its Martin-Baker Mk.1 ejection seat, becoming the first British pilot to use the apparatus in a "live" emergency. It was fortunate that he was alone in the aircraft as the second crew member was not provided with an ejection seat.