UK citizen - Hong Kong - Guangzhou - Macau - Hong Kong

Unfortunately, it is impossible to use visa free transit in the way as you have planned.

The only way how could you become eligible for 144 hour visa free transit in Guangdong province is to fly from Hong Kong to Guangzhou or from Macau to Jieyang Chaoshan airport (sometimes called Shantou airport). By using an air transport you become eligible for 144 hour visa free transit and you can return back to Hong Kong by ferry or train or to Macau by ferry. Anyway, you should have in mind that the eligible itineraries are:

Hong Kong - Guangdong province - Macau

or if starting from Macau

Macau - Guangdong province - Hong Kong

So, you can do this, but only in the way as I have described it. Please, keep in mind that you will need a ticket for either a train or a ferry to Hong Kong or Macau already in your hand before your check-in in Hong Kong or Macau in direction for Guangdong province. You can book your ferry tickets through CKS - Chu Kong Passenger transport company if you will travel to Hong Kong or through TurboJet ferry website if you travel toward Macau. You cannot book your tickets in person, but through the mentioned websites because it is required that there is your name on an e-confirmation that you will receive after a booking.

Also, if you intend to use a train between Guangzhou and Hong Kong, you should have in mind that the only train you can use is a through train departing from Guangzhou East to Hong Kong Hung Hom railway station. You have to book this ticket through an agency (a website like this one - Travel China Guide), again to be granted an e-confirmation with your name on it. If you need some further explanations, here we are.

By the way, 72 hour visa free transit has been replaced in Guangdong province by 144 hour visa free transit in May of the last year.

Thank you so much! You are really helpful and kind. Have a great day!