Hello! Have a long transit-waiting from 05:20 (in the morning)and 20 hours before flight to London in Beijing.

you should have read this web page more carefully before submitting your question. right at the top there is this info:

From Jan. 1, 2013, passport holders of 45 countries and regions will be granted a visa-free stay of up to 72 hours when taking an international transfer via Beijing Capital Airport.

https://www.travelchinaguide.com/embassy/visa/free-72hour/

So, if you don't have a Chinese visa, you will be granted a 72 hour transit visa if you are a passport holder of the 45 designated countries. You could take the Airport Express to the city and explore on your own, but be mindful of the traffic jams in town when deciding on a time to head back to the airport.