The ability to take indirect flights to China has only just opened up in the past week or so, the last year+ it's been direct flights permitted only. Which as you note makes it very difficult to fly from the UK which currently has no direct passenger flights and hasn't had since Jan 21 when Alpha kicked off. As it's the local embassy that seems to set the rules for who is allow on the direct flights to China and gets the health code approval, I don't think there is much experience yet of how things really work yet for connecting flights.

Throw in the strict limits on the number of international flights with passenger numbers limited (social distancing enforced) plus the fact that if an airline transports a flight that turns out to have more than 5 passengers or 5% of the passenger total who subsequently test positive for COVID then the airline is put in the sin bin for 14 days and all flights are cancelled with no notice, and you can start to appreciate some of the very real challenges.

China wants to make it as difficult as possible for people to go to China at the moment, the authorities are just about tolerating some limited movement of non-Chinese nationals through the border, but do not expect your journey to be easy or trouble free. Once you get there expect sub-optimal quarantine 'experience', along with ongoing PCR testing by an army of snowmen, health code 'fun', and a population who are more circumspect of foreigners than usual, it's not for the faint hearted.