London to Hong Kong- fly to Guanzhou- train Hong Kong- Hong Kong to London

No, this route is not eligible because this is a round trip and not a transit.

If you allow me, I will suggest you just a tiny modification of your itinerary. Instead of Guangzhou to Hong Kong by train, you should take a ferry from Shekou port in Shenzhen to Macau. Like this you will satisfy the rule of 144 hour visa free transit that clearly states that the immediate points before and after the mainland Chinese destination have to be in two different countries or eligible territories (eligible territories are Hong Kong, Taiwan and Macau). If you take a ferry from Shekou port in Shenzhen to Macau, you will be perfectly within the rules. Also, as soon as you arrive in Macau, you can take the first ferry to Hong Kong.

In order to do that, you will have to buy a ferry ticket in advance, even before you depart Hong Kong through TurboJet website and print that ticket out in order to have it at hand and ready to present to relevant authorities. If you do that, you are ready to go and to enjoy your 144 hours valid for the entire Guangdong province.