Hi! I have a UK passport and am going to fly Dubai-Beijing-Tokyo.

Yes, you can transit in HK. There is no problem.

It's fine as long as you don't fly Hong Kong-Beijing-Hong Kong. That would need a visa, even if you are staying in the airport.

Thank you so much for the answers to my last question, they were very helpful. Now I am worried that the airline/ airport immigration won't allow me on the plane to Beijing without a visa. I called the airline and they didn't have a record of these changes on their system. Has anyone any suggestions about how to get round this? Thank you again!

It's almost always pointless to call the airline. Call centre agents won't know, and it's not their job. The check-in agents at the airport will know the rules, or have access to them. Transit without visa has been around for many years now, and hundreds of people use it every day. It's nothing special and nothing to be worried about. Just print your onward ticket and confirmed accommodation (if any) and be ready to show them at check-in/boarding and on arrival. If you want to be totally sure that your itinerary is ok, then post it here with every flight and the dates.