The risk Bank of England doesn't like comes with the territory, and everybody who uses public cloud is subject to it. The only difference is that smaller companies don't have a big megaphone or the clout that they have, and must either accept the terms and shut up or find an alternate solution. If Bank of England wants to use their power constructively, they can literally go shopping for data center and IT companies…
The risk that they're concerned with is if every bank is using AWS, then AWS goes down, the entire UK economy crashes until it's back up. Even if AWS itself is more reliable than every bank's on-prem solution, that's no good if it goes down for everyone simultaneously and I can't just use a backup credit card.
They're likely only looking for ways to mitigate these risks, not necessarily a complete return to self-hosting.