Is my itinerary eligible for a transit visa？

You will need a visa as this is a trip to Guangzhou and not a transit. It's viewed as Hong Kong > Guangzhou > Hong Kong. You need to continue to a third country before returning through Hong Kong.

Thanks for your response,
Do you think it would qualify if I do that entire journey on one airline? As there is no direct flight from London to Guangzhou, either way I would have to change flights in Hong Kong. Would Hong Kong then qualify as the “third country” (staying in HK for a few days) before returning to London?

It doesn't matter if you fly on one airline or 2, it's still no transit and you need a visa.

btw. China Southern is flying nonstop from London to Guangzhou.

Why would it not be in transit?
I will not be leaving Hong Kong airport the first time but just to change flights to go to Guangzhou.

Thanks for the suggestion but there’s no direct flights from Guangzhou to HK with China Southern nor will I be able to fly directly back from Hong Kong to London.

Maybe I haven’t been so clear, essentially I want to fly from London to Guangzhou to Hong Kong back to london, Cathay offers the easiest way of doing this but there is a stopover of about an hour where I have to change flights in Hong Kong to go to Guangzhou, does this change of flight without leaving the airport now stop me from qualifying for this?

Even if you don't leave the airport it still counts as a stop. Just the act of the airplane wheels touching the ground disqualifies you. Sorry

This itinerary obviously requires a visa.

You can fly London - Amsterdam - Guangzhou - London with China Southern.

That would qualify you for the 72h twov.