HK-Kunming-Shanghai-London

Julien, to make use of the 72-hour free transit, you can't enter two mainland cities, so I'm afraid this policy is not available to you.

But can I still have a flight from Kunming-Beijing/Chengdu-London since there are no direct flights from Kunming to London? I will just stay inside the Beijing/Chengdu airport? Thank you very much!

Julien, even so you still can't make use of the 72-hr free transit. My suggestion is to try to control your layover time in mainland and see if you are eligble for the 24 hour direct transfer policy. It allows you to enter different cities and the 24 hours are calculated from the midnight following your arrival, which means you can actually stay longer than 24 hours.

Does Kunming adopt this midnight after arrival rule? My apologies but I read somewhere that not all cities adopt this rule. Thank you for all your help!

Yes, I think Kunming adopts this rule. For 24 hour direct transit, almost every city follows this calculation method. If you still have doubts, you can contact the Kunming Immigration for confirmation.