Well that’s one way to spend a lot of money. Azure is not cheap
They’re probably not paying the prices you or I would pay - I would bet that something like this will have gone out for bids so I’d be surprised if they did anything other than pick the lowest number that came in.
The technology they are currently using seems, for want of a better word, clunky.
Virgin Money had a really bad system which dated back to the Northern Rock days before the merger with CYBG and the Clydesdale/Yorkshire system never ran that well either.
I imagine merging it all is a big job, but necessary if they want to tidy things up post-merger and reduce their IT complexity.
This is probably a well good move for them, even if it does end up as expensive, and will allow them to better compete with newer challengers.