flying London - Guangzhou - Auckland & will be in Guangzhoufor 8 hours. Can i leave the airport?

You can apply for the 24h twov and you are allowed to leave the airport. You need to print your confirmed Guangzhou - Auckland ticket and show it at check-in/boarding in London, and tell them you will be in "transit without visa" (it's not a visa). When reaching immigration, go to the dedicated line for free transit which will be clearly indicated and fill out the paperwork.