I do think that your complaint, while applicable to a lot of websites, is not specifically applicable to GOV.UK. They do everything to use plain HTML (and if possible no Javascript at all, but there are certain pages where Javascript is an absolute must like interactive maps), and they aren't using React but instead a special framework that is designed to minimise Javascript even to novice developers. Basically they've replaced jQuery with plain Javascript since that some older devices they're supporting (which jQuery is a better solution as it's bug-tested and optimised as much as possible) is dead.