The UK is an island nation that's reliant on undersea cables. If those cables are cut (which is something that Russia seems to threaten) your only fallback is satellite connectivity - so protecting that connectivity is pretty key to ensuring e.g people can access their money.

I would guess that was how Wes Streeting was briefed (and then completely bungled in public comms).

ideally British banks would have their infrastructure located in Britain and therefore would not rely on undersea cables