Hong Kong Guangzhou, Guangzhou to London (via Dubai)

I have a UK passport and an HK permanent resident ID card. I am planning to fly Hong Kong - Guangzhou, spend around 30 hours in Guangzhou, then fly from Guangzhou to London (transit in Dubai). Do I qualify for the 72 hour visa? Is it better if I exit Hong Kong using my British passport rather than my HKID or does it not matter?