Can't the UK disclose how we know it's China?
They said it's state-sponsored. I'm sure they have a way to figure this out, but it's a pity if they can't tell us what it is. China is saying "you can't just trust governments to tell you these things" and it has a point. What we need is a more concrete public statement of exactly why we think the state was behind it.
(Anonymous because I don't want to be killed by a random Chinese vigilante if I upset them)