I've done this recently - I was already HSBC Premier in the UK, and opened an HSBC China account before I left. It took a bit of effort, but I think it's still easier than opening a bank account once you get here. A few things to be aware of though:

• Opening the account from the UK required a ~40 minute phone call and a half hour face-to-face appointment in the branch. They ask a few difficult questions like how much are you going to be paying into the account per year, but I don't think it actually matters what you say, so don't worry about it too much. (I've actually put in about 10% of what I told them, but nobody has complained.)
• They said that they'd send my China UnionPay debit card to my address in the UK, but it hadn't arrived after about 2.5 months. So once I got to Beijing, they cancelled that one and sent a new one to my Beijing address, which was fine.
• They sent a little plastic HSBC Secure Key to my address in the UK, which arrived fine. You'll need it for some online banking (especially when logging in for the first time, IIRC).
• You need to nominate a specific HSBC branch in China to go to to pick up your PIN, once you have your debit card. So check which branch will be most convenient for you before you apply for the account.
• You end up with two linked Chinese bank accounts - one in your home currency (GBP for me) and one in CNY. The idea is that you pay from your home HSBC account into your China account with the same currency, and then convert it across into your CNY account. HSBC seems to sneak a fee onto the GBP-GBP transfer though, so I just pay from my GBP Revolut account into my China GBP account, fee free.
• You can only do FX transactions (i.e. between your GBP and CNY accounts) during Chinese "working hours" (9:30am-6pm Monday to Friday). If you're only converting small amounts at a time, you need to make sure you "stock up" before the weekend!
• The loss on GBP -> CNY transfers between my two HSBC China accounts is about 1% off XE.com rates. When you make the FX transfer, you have 15 seconds to accept or decline the rate.
• After everything is set up, your account won't work for AliPay or WeChat Pay until you activate your UnionPay card for online purchases. For me, I went to my local branch in Beijing and signed a couple of forms - the guy there spoke reasonable English and knew what to do.
• If you're new to the Chinese payment systems (i.e. WeChat, AliPay), they need "real name verification", which requires details to match exactly with what you gave to your bank. Something to be particularly aware of if you have multiple passports (i.e. always use the same passport details that you have registered with HSBC, even if you enter China on a different passport). Likewise with mobile phone numbers - for banking purposes I still use my UK mobile number, even though I now have a Chinese number too. Having a dual-SIM phone really helps... And my "Real Name" for these purposes is what's printed on my HSBC China Debit card, which is [Last Name] [First Name] [Middle Name].
• I've never tried converting CNY -> GBP, so not sure how that's done. As you'll have seen from this forum, it's a lot harder to transfer wealth out of China than it is to bring it in. I assume it's possible somehow though...
• The HSBC China Android app is a bit of an odd one - there used to be a version on the Google Play Store (not sure if it's still there), but it wanted updating as soon as I downloaded it, and directed me to a Chinese app store for that, but it was simple enough to follow, even for a non-Chinese speaker like me (just hit the obvious big blue buttons). Once the app is installed on your phone it's pretty smooth (and mostly in English), although you'll still need your physical Secure Key for some transactions. There's no digital Secure Key feature yet like there is in the HSBC UK app.

Hope that's all helpful, let me know if you still have any questions about using HSBC in China!