Unfortunately, that was in the 1950s when the UK was spending about 10% of GNP on Defence, compared to 7% in the 1960s.
They were also cheaper to develop and cheaper to build than the generation of aircraft that replaced them.
I've also got it in my head that the USA paid for some of them through MDAP. Can anyone confirm that?
You didn't mention the V-bombers, but I think the USA paid for some of them either directly or indirectly.
Also the UK was desperately short of Dollars after World War II. Hence Austerity and the Export Drive. The Treasury didn't have the Dollars to pay for American aircraft even if they were better and cheaper than what the British aircraft industry could make.
And before anyone says, "Then why did they buy 52 Lockheed Neptunes?" The answer is that they were provided via MDAP and IIRC ordered in the first place because Avro couldn't build Shackletons fast enough.
Although they were built in Canada, rather than the USA, I think that the 400+ Canadair Sabres were paid for through MDAP. Can anyone confirm that?
A few crucial points in there.
Yes, the story of British defence in the first half of the Cold War was one of declining defence share of GDP and manpower, not cash per se.
Yes, the design of a 50s sub/transonic day fighter is vastly different from a 50s All-weather fighter, which is why the Swift/Hunter/Scimitar are so different to the Javelin/Sea Vixen. When you jam both of these requirements into one plane then make it supersonic you have 3 development drives which dramatically increase the cost and the consequences for getting it right/wrong.
IIUC the US paid for Valiant development and production via MDAP.
Yes, the British had the Sterling Area where they could trade in Sterling without impacting on the scarce Dollar reserves. That's why they instituted the Joint Project with Australia and other Commonwealth countries for nuclear and rocket development.
It wasn't just Neptunes-Shackeltons, the same thing happened with FAA Skyraider AEWs and Gannet AEWs, as soon as they could the British replaced American aircraft with British aircraft for what I call 'whole of government' reasons. People are far too fixated on speed and unit cost of aircraft, its no point having a lot of fast planes that you can't get parts for because there are no US dollars to buy them with.
I don't the details of the Canadair Sabre deal, but I'm guessing the trade relationship with Canada was significantly different to the US which made the deal attractive.