You had me until “4 billion”.
Those guys clearly have no idea how much this should cost in terms of development time, so instead they will give 4bn to a company that’s happy to con them and give them crap in exchange, which at best is still going to be “legacy” compared to real innovation like Monzo and Starling, and at worst will cause a TSB-style disaster.
They can actually modernise their business with much less by building an in-house tech team and simplifying their product line (the less products the less code & edge-cases need to be handled).