How do you report a foreign bank or brokerage account UK

So long as you declare them correctly on your Self Assessment, you're all good. There's no need to do anything else.

However, if you are a tax-evading scumbag and not declaring offshore income and gains, a few years ago (~2017/2018) HMRC - as part of a long overdue "Requirement to Correct" crackdown on such behaviour - put some "disclosure" facilities (the "Worldwide Disclosure Facility" and "Digital Disclosure Service") in place so such slimeballs could try and get their tax affairs back in order. More on this here: https://www.gov.uk/guidance/offshore-disclosure-facilities , but they're really only relevant to folks who haven't been paying offshore tax correctly.

As that page notes "New international collaboration to share information and laws to tackle non-compliance has started", meaning HMRC is probably already well aware of any foreign bank or broker accounts you hold, unless you're knowingly committing fraud to bypass foreign accounts' "KYC" (Know Your Customer) processes, or using dodgy providers.