According to the Register, it's Lloyds Bank, Halifax, TSB, Nationwide, First Direct, Bank of Scotland, and Barclays.

Interestingly, all the banks seem to have the exact same notice on their websites about this. (See https://www.theregister.com/2025/02/28/payday_from_hell_as_s... )

None of the reporting on this seems to report a cause for what might be behind this. A concerted DDoS campaign? Failure of some shared common facility? Nation-state-level hacking?