Transit 6 hours in Beijing airport from Thailand to London with Air China need visa or not?

The most important detail is that you will travel through China (Beijing in your case) covered by 24 hour visa free transit. So, you will not need any kind of visa. Those Thais and others who say that it is not possible to perform a transit in Beijing are wrong. They confuse the transit with the possibility to exit the airport. So, transit is granted, but the exit from the airport for sightseeing can be refused, especially for those having less than 8 hours of layover.

This visa free scheme allows you to travel visa free via Beijing and there is no doubt about it. Your journey is eligible for visa free transit scheme. Also, you will see that nobody will make you any troubles, especially since you are traveling on Beijing based Air China. Even if you were not traveling on Air China, there is no any single reason why would any citizen of any country transiting in Beijing being refused. You are good to go!