https://www.onr.org.uk/publications/regulatory-reports/other... is an interesting read; the ONR claims it was not them, and EDF/Framatome claim it was not them either.

The Fingleton Review goes into this and other design changes required by other agencies. https://www.gov.uk/government/news/taskforce-calls-for-radic... .

Sizewell C has an approved design, so the risks are much reduced, and the political/regulatory environment is behind simplification.