I am flying from London Heathrow to Bangkok with a transfer in Shanghai do I need. Bisa

Sarah, your situation is very simple. The first and the most important detail is that your itinerary will be covered by 24 hour visa free transit and you do not need any kind of visa.

If your layover in Shanghai is a (relatively) short one, up to 8 hours, you will not exit the airport (transit area), but you will literally just walk from one to the other gate. You do not have to perform any formality and you do not need to fill out any form. Also, you will not be a subject of immigration and customs control. In reality, very easy and straightforward situation.

If your layover is a long one, longer than 9 hours, you will maybe want to exit to the city. In this case, the first thing you will upon landing in Shanghai is to approach 24/144 hour visa free transit where you will apply for a temporary entry permit by presenting your flight tickets. After a short check, your permit will be granted and you will head to the immigration and then to the city to enjoy some time on your own. This application is free of any charge. Basically, this is it. Have a nice journey.