Exaggeration: everything on the grid has a "trip" for instant disconnect, and there's usually a steam bypass for ramping down fairly quickly.

Ramping up takes longer, yes, especially from maintenance. But the real factor is that operating cost is constant, almost independent from output, so they want to run the plant at operational 100% as much as possible.

(handy UK status page: https://www.ukgridlive.co.uk/nuclear )