Answered by ZUPAN | Dec. 17, 2019 10:13
Susan, if your flights:
Phuket - Shenzhen
and
Shenzhen - London
Are both non-stop and you don't have any additional landing within mainland China, you are definitely eligible for 144 hour visa free transit in Shenzhen and you don't need any kind of visa.
You will print out your flight tickets and bring those printouts with you. Also, you will declare at check-in in Phuket that you will travel to London via Shenzhen using 144 hour visa free transit. After a short inspection of your tickets, you will be allowed to board the plane.
Upon arrival in Shenzhen, you will approach 144 hour visa free transit counter where you will fill out the blue form. Shortly after, you will get a sticker in your passport any you will be allowed to exit the airport. Everything visa free and everything free of charge.
You are advised to book your accommodation in Shenzhen in advance and to bring a confirmation (a printout) with you because you will encounter a several questions about your accommodation when filling out the blue form.