It's much more than cables. You'd need an entire separate internet - ISPs, BGP, DNS, TLS PKI infrastructure, and probably many others. You'd also still need to very carefully keep this separate, as simply connecting an Internet and a "UK Power Plant Internet v2" cable into the same router or even laptop might invalidate all of the isolation you spent time building. Which is itself a losing battle, given that you need everyone's house to also be connected to this network.

You, of course, then also need to ensure redundancy on this "UK Power Plant Internet v2", as you don't want to have a single point of failure such that a single router or cable somewhere being interrupted would create a network partition.

And again, there is simply no one even considering this. All of the work going into smart grid coordination (strictly necessary for incorporating a large amount of renewable power generation in the grid) is 100% designed to work on top of the public Internet.

A VPN is not some foolproof system that guarantees you can't be hacked. If you're connected to the internet, people can probe your systems. Heck, Stuxnet proved they can do it even if you aren't.

I don't know about power plants, but for critical water infrastructure, there's usually not even a building for someone to sit in 24/7 and at least 99% of the time there would be nothing for them to do.

It's likely that much of this stuff is still running on Windows XP which doesn't even receive updates. But of course it's still exposed to the internet.