LONDON > GUANGZHOU > HONGKONG

No, unfortunately, you cannot. Train from China to Hong Kong is not allowed under any visa free transit scheme. Also, it is not possible to travel from Guangdong province (Guangzhou city) to Hong Kong overland under 24 hour visa free transit. The only way how you can travel from Guangdong province to Hong Kong under 24 hour visa free transit is by plane from Guangzhou to Hong Kong.

However, I would advise you to rely on 144 hour visa free transit if you are UK citizen. To be able to use 144 hour visa free transit, you will have to stay at least a minute longer than 24 hours in Guangdong province and then leave it by ferry. You can use ferry from, for example, Nansha port located south from Guangzhou and that is easily accessible my metro. If you decide for this option you will have to buy your ferry ticket even before your departure from London through CKS - Chu Kong Passenger Transport Company, print it out and have that printout at hand, ready to be presented to relevant authorities. Also, you will have to take exactly that ferry you have bought your tickets for. In the end, I will again tell you that you cannot take a ferry that has less than 24 hours distance from the scheduled time of your plane landing in Guangzhou.