Do i need i visa?

I Saw A lot of your answers about that i won't to need visa, but i just want to make sure

I’m planing a trip London-Chengdu- Bangkok and back Bangkok-Shenzhen- London.

My transit in the Mainland China is less then 24 h and I’m planning to stay within the transit area.
Will I need any visas to stay at the airport got the duration of the transit. I found the information that Chengdu is visa free but one website mentions that SZ doesn’t respect the 24h none transit visa policy. This is the info from January 2018. Your website from 2019 mentions that this airport is visa free.
Please advise so I can prepare my journey in advance.
Thank you