I think to be fair some of it was also due to stuff done in the early days of figuring commercial power out; but that was a long time ago. Sellafield has taught a lot of people what's needed to do it properly.
Even that was using technology which haven't seen much use outside the UK. They were never truly commercial, supporting the nuclear weapons program was a requirement from day 1 and was the reason for some design choices that was suboptimal for power generation.