How much baby milk powder can I bring from the UK to China? And do I need to declare it at customs?

You can take the enough milk powder for your babies as personal things, but the price of all your personal things cannot be over 5000 RMB.
As for the milk powder, if you take more than you can use, you should declare to the custom and be charged the duty. If a jar of milk powder is less than 800 grams, no more than 24 jars are OK, but more than 800 grams, you can not take more than 6 jars. If you take more, 10% tax will be charged.

If the jar/carton is exactly 800g, then how many cartons could I bring from the UK to China?

Then you may be allowed to take in 6 cans. According to the regulations, you can take at most 5kg milk powder in.