Your outline of the BP Delta is very brief and thus misses the critical feature, that of total hydraulic, irreversible, powered flying controls. Artificial feel for the pilot was by way of built in springs which could not be fully trimmed to take into account the very fine degree of control necessary, particularly with the minimal input demanded at high IAS. The design work was carried out at Farnborough, and all of the early flying was by Aero Flight pilots including Bob Smythe CO, Jim Harrison, and Jock Elliot. Jock carried out most of the flying with 52 flights totalling some 26 hours. He it was who, commenting on the sensitivy of the controls at IAS, stated that, at 500 kts IAS, 1/4" stick movement gave 1"g" of acceleration, and observed that the possibility of a strong reflex input was very real and would have catastrophic results. Following a belly landing by Jim Harrison the design was near to being shelved, when Jock came up with the idea for variable gearing between stick and surfaces; this facility would be available to the pilot in flight and would allow a choice ranging between 1:1 to 4:1. Jock pointed out that the idea was not new and that some of the Schneider Trophy aircraft had incorporated the idea.
The idea was discussed at a meeting with Dennis Higton, aerodynamicist at RAE, JD North of BP, and Jock, and the outcome was that it was incorporated into the design, now known as the BP P111a.
The point has been made (Dave Morgan) that Jock's concept of variable gearing was an crital break through for the aviation industry and virtually paved the way for Concorde and all high speed aircraft that followed.
Ben Gunn flew the aircraft in it's later form of P111a.
The Delta was a significant, and brave, design at a time when even swept wings were viewed with some trepidation, and the delta was truly futuristic; the variable gearing was crtitical in the design of all high speed flying from that point on.
I hope these notes add to you knowl...