UK passport holder travelling direct from Bangkok to Beijing then direct to Kuala Lumpur

John, it seems that you have some leeway when using the 72-hour free transit policy. In Bejing, the 72-hour period starts from the moment you have done all necessary formalities at the immigration counter, so if there is a long queue, it may take longer to process all things. However, you had better confirm this with Beijing Immigration in advance.

As long as you go through Immigration with the 72 hour time limit. There is a question of when the time starts. However things like this are often dependent on location, and overall policy. If there is any doubt whatsoever, check and check again upon arrival - ask that the specific day and time of expiry be written down on the visa itself. Exceeding the time limit makes you subject to delay, questioning, a fine, and being put on a watch list.

If you are in any doubt whatsoever, ask that the specific date and time be written on the entry permit itself. As long as you pass thru Immigration with the 72 hours, you will be ok.

It is unwise to violate the time limit - to do so makes you subject to delay, questioning, a fine, and being placed on a watch list.