It's OK to do most things with chunk of normal servers, but when you need to handle very large number of transactions with fast commits, and stuff like eventual consistency is not allowed, it becomes expensive to handle them no matter what.
One of the other big Australian banks (Westpac) has an "exit IBM" project as well, but it isn't complete yet.
There is no information anywhere saying that they moved their critical banking systems and transaction processing.
Unisys always had a smaller mainframe share than IBM, and they stopped making new chips sometime in the 2000s.
For at least ten years the fastest Unisys mainframes have been very big (proprietary) x86 machines running Windows with a Unisys mainframe emulator.
For some time I defined a "serious computer" as something that didn't have ports for keyboard and monitor.
Having said that, the pre-x86 A series were pretty cool. And ran the most user hostile OS ever created, to the point its very name was used for Tron's villain.
Monzo? They're getting bigger now, circa 3M customers. More than First Direct, Starling, Metro Bank.
I personally worked on a project there that involved building new virtualization platforms for not only x86/x86_64 Intel, but also AIX (Power) and Solaris (SPARC).