If China can cause a denial of service attack on a country by remotely bricking all the network infrastructure or even slowing it down to a degree, this would still be economically devastating at the least.
I think there's a difference worth noting between subtle monitoring & coersion vs. a full out act of war. By the time China is trying to denial of service the UK's 5G infrastructure we've got other things to worry about.
But they don't need to dos the whole country, just interfere up to the point of plausible deniability. They could do targeted outages for some UK firm at a stratetic moment or something.
Also in the case of an actual war, it's surely better to not have your entire nations communications under enemy control.