They've already been trying all of that, it's been happening since the war started.
Many UK citizens have been involved in the war for a long time. There is also a huge cyber force of civilians involved in hacking from our side too.
We are actually less of a target than Eastern European countries because our defences are stronger. But all ISPs and everyone else involved behind the scenes has been strengthening and mitigating more since 2022, on a continuing basis.
It's not actually very easy to hack things in a substantial way at all. You can cause minor disruptions to things, but honestly companies making their own mistakes do worse every day (just look at the airlines, Microsoft, etc.).
Plus they have to divide their cyber warfare resources between a lot of different locations, they only have a finite amount of manpower.
Water would be a silly thing to target, it's about the only thing we have in abundance.