The pound is not just a currency, it’s a symbol of British sovereignty and independence. Will the UK be willing to trade the queen’s face on their banknotes for Euro symbols?
You're assuming we still even use cash at that point. I only have physical cash in my pocket once in a blue moon these days. I rarely even get my card out since contactless on phones came in. Therefore I prepose a compromise that Google wallet is updated to show a picture of Charles giving the thumbs up while playing the opening bars of Rule Britannia as it's new payment confirmation.
It's still necessary in lots of places though, for example village fetes etc where there are no reliable contactless systems. I've been to events where they have card readers paired with phones but the local masts get overwhelmed with the number of people and the phones can't get a signal, even when they have brought in mobile masts for the occasion. So unfortunately you still need cash.
I mean, I you have even the slightest degree of impule control and the ability to do maths in your head, it's pretty easy to control your spending on card too.