What was the Jaguar originally designed for on the British side?

The British Air Staff Target 362 called for an advanced supersonic trainer. What the United Kingdom was looking for was an advanced supersonic jet trainer to replace the Folland Gnat T1 and the Hawker Hunter T7.

France, by contrast, needed a cheap, subsonic, two-seat dual-role trainer also capable of light attack work, to succeed the Fouga Magister and the Dassault Mystère IV. In May 1965 the two countries signed a memorandum of understanding to develop two aircraft: a trainer based on the ECAT and the larger AFVG. Negotiations on both sides of the Channel led to the creation of SEPECAT. SEPECAT (Société Européenne de Production de l'avion École de Combat et d'Appui Tactique) was set up in 1965 as a joint venture between BAC and Breguet.

The aircraft's requirement later changed to include supersonic performance as well as reconnaissance and tactical nuclear strike missions.