How does that compare to the operational cost of developing an app, maintaining an app, paying for the infrastructure behind it, and having the payment processors take their cut, operate customer support etc?
On top of that they'd need to pay their employees more and of course handling cash comes with security and, inevitably, enforcement. Additionally, you need people at every place where cash can be used as opposed to having almost all public transport stations unmanned like in Italy.
So let's say ~double that and round to 600 million to a billion GBP per year.
You can do some serious app development with that I would think. And because they don't pay the app store tax it's going to really be close to that amount.