Two factual misconceptions on the thread I would like to correct (I'm not getting involved with the arguments, lol)
First, the P.1154RAF was single engined, by the time of cancellation the P.1154RN had evolved into a two seat *twin engined* design in which the nozzles were cross linked to avoid assymetric thrust loss. ie one engine exhaused out of the front port and rear starboard nozzles and the other one vice versa via a complex overlapping ducting arrangement. This model was a disaster waiting to happen and it was right it should go. The simpler single engined RAF model however *could* have proceeded successfully.
The P.1121 did not use the same engine as the NA 39. The Gyron Junior was the engine in the NA 39 and in the P.1121 it was the Gyron, despite the name, it was a very different and much bigger engine. How successful that might have been can never be known but the failures of the Junior should not be used as a yardstick.