Just as you would buy a non-contract SIM from a MVNO like Lebara or Lyca or Virgin in the UK, you buy this one from CMLink which is brand of China Mobile in the UK. In the UK it operates on the EE network, but in China it uses China Mobile. You should order it a couple of weeks before you travel. You must activate the SIM in the UK before you leave.

If you take for example the £11 40G bundle you would need to top £11 and then send the appropriate message to the SMS short code to convert your £11 credit into the 40G bundle. This gives you 36G of data in the UK and 4G of data to use in China, plus some minutes to use in UK and some to use in China. The SIM card has a UK number, unless you opt to also have a Chinese number at extra cost (I don't see the point for your short trip). Activate the SIM card and it's bundle a few days before you leave the UK and just make sure it's working, check your bundle using the word "check" to the SMS short code. Then make sure you have roaming enabled on the SIM in the phone and it will just work when you arrive into China. Any data you use is whilst in China is tunneled back to UK and will break out onto the internet in the UK so you can still access the socials, BBC news and radio, etc.

The bundle will try to renew after a month so unless you expressly selected options for auto top-up if there is no PAYG credit balance then the bundle can't renew and that's it.
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It's a hassle free option that eliminates the need to faff with VPNs to get to Gmail, etc.