It seems that 1957 was a critical year for the United Kingdom, as that is when the infamous White Paper devastated the British aerospace industry.
Problem is 1957 is when they finally grasped the nettle to do something about it, the problem goes back to ~1940 or so. The British developed a truly vast aircraft industry during WW2, one far bigger than they could sustain in peacetime. Rather than cutting it back immediately, they gave in to political pressure (it being a vast employer) and kept it going until they ran out of money. Remember, by 1957 10% of GDP was going to the armed forces and something like 1/3rd of R&D expenditure was for them. That's fundamentally unsustainable, something had to go and Sandystorm was it. If they'd accepted the need for cuts earlier, they would have been substantially less painful - but the aircraft industry didn't want to get off the gravy train.
The next year the United Kingdom signed an agreement in which it essentially adopted United States military naval reactors and nuclear weapons.
Realistically I think they had little choice. With naval reactors the problem was one of enriched uranium supply and the manpower to develop a reactor - the British had already independently decided to go the PWR route, but without access to US stocks of enriched uranium and reactor designs they wouldn't get a boat to sea with one until the 1970s.
So far as weapons go, the US designed weapons were incomparably safer and significantly lighter. Adopting US designs in the circumstances was a no-brainer.
1958 is also when the United States signed the EURATOM Agreement with Western Europe, which resulted in widespread adoption of American light water reactor designs, closing a major potential market for British and French gas cooled designs.
In a world where uranium enrichment is cheap and you get large numbers of nuclear submarines, it's hard to see PWR not becoming the dominant technology over time. Gas-cooled reactors have some really nice benefits, but it's very hard to argue with the safety benefits that the enormous number of PWR operating hours bring - and that in large part comes in the early days from their use on submarines.
More crucially, building with export in mind would have helped British industry in general.
Maybe, maybe not. It's instructive to look at the French defence industry here - their products are structured around being exported, which leads to the French armed forces by and large getting slightly inferior kit to the UK which puts less of an emphasis on export. It can be a useful discipline, but concentrating on it too hard can lead to a race to the bottom with all sorts of nasty consequences for industrial capability.
Gas cooled reactors don't require expensive and difficult to build pressure vessels like pressurized and boiling water designs, and they offer higher efficiencies and the possibility of using natural uranium. The early gas cooled designs probably were just as efficient as PWR due to their use of steam turbines instead of gas turbines, but gas cooled reactors can use gas turbines, which are more efficient than the steam ones that light water designs use.
Gas turbines are very rare (I'm only aware of PBMRs using them, and they've got a Helium coolant), a steam cycle on the secondary cooling loop is much more common. The most likely early gas coolant (Carbon Dioxide) is IIRC activated as it passes through the reactor meaning the turbine has to be within the bioshield, and it's something likely to wear out. Helium is much less susceptible to activation if at all (can't remember), making that a less severe problem.
