Hello. I'm planning to visit Chengdu and will fly in and out of HKG.

Your route back is like this: Hong Kong - Chengdu - London, right? If yes, you can take advantage of the 72hr rule when transiting in Chengdu.

Thanks Betty!
My complete travel journey is STR-LHR-HKG-CTU (stay 2 nights in Chengdu), then back to HKG-LHR-STR. I had the Impression the 72hr rule does only apply if you're transiting thru China e.g. LHR-China-LAX? But is not valid for a journey like LHR-China-LHR? Am I missing something here? Much appreciate your comments!

Yes, your undertanding is right. You can't fly in from a country and fly out to the same country under the free transit rule. As your way back is via Hong Kong, I think this disqualifies you for the 72h free transit.