I would like to know whether I need a transit visa or not.

Your itinerary is definitely and undeniably eligible for 24 hour visa free transit and you really do not need any kind of visa because flying on separate tickets and on different airlines is allowed! You will just print out your flight tickets and have those printouts at hand.

On your flight from UK to Shanghai you will get arrival/departure card and you will fill it out and as a purpose to your visit to China, you will tick the box - transit.

Upon your landing in Shanghai the first thing you will do is to immediately approach transit counter where you will apply for a temporary entry permit by presenting your flight tickets and your passport. This permit will allow you to cross the immigration because you travel on separate tickets, so you will have to collect your luggage and perform new check-in. This permit is free of any charge and the entire procedure is more or less a formality. After you have crossed the immigration you will continue toward your onward flight for Thailand.

All these details are presented in Timatic that is used by airport staff in determining if someone is eligible for a travel through certain countries. You are definitely fine and again, you do not need any kind of visa, but you will just repeat that you fly from UK to Thailand through China using 24 hour visa free transit. That's it.