Martin-Baker 5
From Link

The M.B.5 (serial number R2496) flew for the first time on May 23, 1944. The performance was superlative, the Armament and Aircraft Experimental Establishment raving about the general design and layout of the aircraft. All pilots who flew it praised its qualities, and it is sort of a mystery why this airplane was never put into production, even though there would have been enough time for this plane to have entered squadron service by the time of the end of the war against Germany.
Martin-Baker M.B.5
Rolls-Royce Griffon 83 inline @ 2,340 hp
395 MPH @ SL
426 MPH @ 6,000 ft
460 MPH @ 20,000 ft
20,000 ft in 6:30
34,000 ft in 15:00
40,000 ft service ceiling
4 x 20mm (Wing)