LHR-HKG-train to Guangzhou/Foshan-CAN-LHR

You can do that, but there is one detail that you simply have to change within your itinerary. Taking a train from Hong Kong to mainland China is not eligible for 240 hour visa free transit, so an alternative is an imperative. Eligible ways how to enter mainland China from Hong Kong are by air or by taking a ferry on a direct line from Hong Kong to Nansha port (located south from Guangzhou and connected to it my metro) or to Shekou port in Shenzhen. So, you can choose between those two ferry lines. Even before you have taken one of the ferries, you will have to have your Guangzhou to UK flight ticket and, please, have in mind that your flight itinerary from Guangzhou to UK is not allowed to have Hong Kong as a layover. It has to be a direct, nonstop flight Guangzhou - UK. When you arrive at Hong Kong ferry port to take your mainland China bound ferry, you will declare that you are traveling to UK via mainland China using 240 hour visa free transit. You will present them your Guangzhou - UK ticket and you will be allowed to board the ferry. Upon your arrival in Nansha or Shekou, the first thing is that you will apply for a temporary entry permit at the transit counter (in reality immigration area) where you will again present your flight ticket for UK. This is all free of any charge. After a short check, you will be granted a permit and after that you will cross the immigration and exit the terminal building to proceed toward Foshan and to other places in mainland China of your choice. When a time for your flight arrives, you will take it from Guangzhou to UK. Also, I encourage you to use correct terminology because this is 240 hour visa free transit and not 240 hour visa or similar. Have a nice visit to China.

Thank you very much indeed, Borut! That is very clear and full of the essential details. Now I must convince my wife that it is not possible to do it by rail! Then look up those ferry routes and times. I imagine it is an interesting journey by boat: more to see and time to see it.

Miles, trust me, taking a ferry is quite easy and here we talk about quite fast catamarans. Both, Hong Kong to Nansha and Hong Kong to Shekou lines are performed by Hong Kong based Chu Kong Passenger Transport Company. The line for Nansha departs from Hong Kong China Ferry Terminal located at Tsim Sha Tsui neighborhood and it takes just 90 minutes to reach Nansha. The line for Shekou departs from Hong Kong Macau Ferry Terminal and it takes just 60 minutes to reach Shekou. It is not a problem at all. Have a nice journey.

Flights are booked. Guangzhou - UK isn’t direct, but transferring in Dubai (UAE, which is on the approved list), so I hope that will satisfy the conditions.

Thanks again, Borut, for the help.