It was months of back and forth before they finally told me that I had offhandedly mentioned my salary and they wanted proof of that. Despite never needing proof before, and despite them being the bearer of my bank account so they could see this. I had to refuse to leave the HQ on Fleet Street for 4 hours before they even told me that.
They wouldn’t accept my payslip pdf as proof. So I walked across the street to Barclays and opened 3 accounts on the spot and never looked back.
Ironically to this topic, I had to close that account when I left the UK because I didn’t have a UK address. But HSBC handled my case really badly, I nearly lost my accommodation because of their opaque stalling (I need to prove direct debit before move-in). So I would never go back.