jamesbelchamber parent
I haven't come across a banking app in the UK that doesn't work with GrapheneOS. HSBC insists you use the AOSP or Google keyboards but otherwise no issues.
Santander at least used to not work, I haven't tried it with the new app they launched. The old app certainly wouldn't work and I was told by customer service there was no way to access it on a phone with an unlocked bootloader.
You are supposed to (and GrapheneOS prompts you to) relock the bootloader immediately after installation of the new OS.
HSBC now even complains if you have Developer Options enabled.