The Submarine reactors research is all done at Douneray on the Northern coast of Scotland, so Barrow and Sellafield is just a coincidence.
The Navy subs visit US&A on a regular basis, for swapping out the ICBMs. So even if they do build them in UK, there must be a reason for this, maybe they need serviced or the warhead is american.
Hydropower is very flexible, some of the dams have the capability to pump the water back uphill when electricity prices are very cheap or negative, dam operators have been doing this for decades to maximise profits. Also hydro dams can go from 0% to 100% in about 60 seconds. The nuclear power stations all have a secure power link to a hydro dam so that if all else fails they always have a reliable independant power source.
The reason UK is uniquely placed in europe for the biggest potential renewable power generation is the gulf stream, this current flows towards the west coast and especially through the Pentland Firth. West of Shetland it flows between 1-4 knots, at the Pentland firth when its has to fit through the gap of John O Groats and Orkney, it can be 7 knots. I was involved with a firm in 2000 that was renting gear to research teams that were trying to build scaled prototypes for generating power, but everything alway broke, the power of water was always too strong. As far as I know the research stopped and the next biggest team was bought out by the Portugese government. The energy density of water is way more than wind, and the gulf stream is constant, 24/7. If this country was serious about being green they would pump some cash into that and would be a net exporter of power in no time.