|Quoting NoUFO (Reply 3):|
Being too complicated was one thing, I believe. It's a single-seater, and the workload must have been enormous. A second seat, however would add weight to an aircraft that was supposed to be a fighter, not (fighter-)bomber.
More precisely, it lost in competition to other designs, such as the Dassault Mirage IIIV and the Hawker Siddeley P.1154 for a NATO supersonic VSTOL fighter. Political disagreement between the UK, German and French governments torpedoed that project after the Hawker Siddeley P.1154 became the lead contender over the Dassault contender as it was judged to be technically superior. The UK government continued development of the P.1154 as a joint RN
/RAF project until disagreement over specifications and requirements between the RN
and RAF killed that project as well.
Mind you, the Dassault fighter had 9 engines (yes, 9!); a SNECMA modified version of the Pratt TF30, plus 8 Rolls Royce RB162 lift engines... a very complex machine.
In the end, all of the engineering and research especially for the P.1154 led towards the Harrier. Development of a true production supersonic VSTOL fighter only came into fruition with F-35B.