Onward country visa at airport

William, if I have properly understood, your itinerary will be:

UK (I assume) - Guangzhou - Cambodia

If I'm right and you don't have any additional stops, especially within mainland China, your itinerary is definitely eligible for 144 hour visa free transit in entire Guangdong province. Please, be informed that this is not a visa, but a visa free transit. Also, 72 hour visa free transit has been replaced in Guangdong province by 144 hour visa free transit in May of this year.

You will just get a sticker and not a visa in Guangzhou.

As far as Cambodian visa is concerned, you should not worry about because Chinese authorities are well aware that UK citizens are perfectly eligible for a Cambodian visa on arrival. You will not have any problems regarding this detail.