I can’t for the life of me come up with a single reason that Russia or China would engage in a direct war with the U.K. The only way they’d bother is if they somehow managed to completely cripple the US armed forces while still retaining their own operational capabilities.
Sure, the cables are at risk. So what? Those are public wires. Every nation of consequence has communications satellites in orbit. I hardly think that the U.K. is actually using cables as opposed to sat communications with your armies.
I guess cutting the cable would me GCHQ and the NSA couldn’t see what pr0n video you are watching but that’s about it.