Long discussion: https://www.worksinprogress.news/p/why-british-nuclear-flopp...

Magnox: un-enriched fuel, designed for weapons, low operating temperature.

AGR: too complicated, can't be refuelled without complete shutdown.

PWR has emerged as the only viable design, and even that is too complicated compared to solar+batteries.

Basically, plumbing is the enemy. Every joint is a potential failure. If what's in the pipe is radioactive or corrosive or explosive (in the case of molten sodium, all three), that's extra bad. So there's tremendous cost of inspections.