UK -Guangzhou - UK

Grant, it is 144 hour visa free transit in Guangdong province and not 72 hours... Also, this is not a visa, but visa free transit, so there is no visa of any kind.

You cannot travel UK - Guangzhou - UK because this is a return trip and you miss a country/territory C to be in a formal transit. So, you will have to insert one country or eligible territory (eligible territories are Hong Kong, Taiwan and Macau) within your itinerary to be eligible. In other words, your reasoning in the end of your post is correct. An itinerary that looks like:

UK - Guangzhou - Amsterdam - UK

or

UK - Guangzhou - Hong Kong - UK

Are eligible for 144 hour visa free transit. You should just be aware that you can fly from Guangzhou to Hong Kong or take a ferry. You cannot take a train or bus (bus in theory would be good, but the entire situation is quite tiresome). If you decide to take a ferry you will have to buy your ferry ticket even before your departure from UK through CKS - Chu Kong Passenger Transport website, print it out and have it with you. Also, you will have to take exactly that ferry you have bought your ticket for.

is it possible to fly:

London - Hong Kong - Guangzhou - Hong Kong - London on the 72 hour visa free policy?

Hong Kong - Guangzhou - Hong Kong is a return trip, not a transit, so not eligible.
BTW, 72-hour has been extended to 144-hour in Guangzhou.