Which research aircraft validated the Lightning's low tail?

The SB.5 was described as a "highly unorthodox" British research aircraft with an adjustable wing, designed by Short Brothers in response to UK Air Ministry requirement ER.100 to provide data for the design of the English Electric P.1, the prototype of the English Electric Lightning, by testing the low-speed flight characteristics of different wing sweep configurations needed for supersonic flight. A single-seat, mid-wing machine was built precisely to investigate the low-speed behaviour of the possible configurations.

The aircraft was designed to accept three different sweep angles: 50°, 60° and 69°. The wing sweep was adjusted with the aircraft on the ground. The entire rear fuselage, just behind the engine, was removable, and two alternative rear fuselages were available. The horizontal stabiliser could be mounted on top of the fin, in a T-tail configuration, or below the fuselage.

The experience gained with the SB.5 validated the wing sweep and the low-tail configuration adopted for the P.1. The original P.1/Lightning design proved an effective configuration for high-speed flight. The P.1 itself, conceived to explore transonic speed and built to specification F.23/49, had an ultra-thin wing with an extreme sweep of 60°. WG760 first flew on the morning of 4 August 1954 from Boscombe Down.