When I was younger (like 16 or something) I bought something off paypal that took several days to be taken out of my balance, and so I spent a little more than I had (£4 more, in fact) and because this is before the days of £10 safety net, and I was too young to have an arranged overdraft, they wanted to charge me £28 for going £4 overdrawn. And, because the charges are based on an arbitrary day of the month that was only 2 days later, they wanted to charge me again if I didn’t get at least £4+28 to them that morning (it was a Saturday and they closed at 1pm). In the end I got them to waive the charges, but they were certainly keen on making money.
It’s like a decade later now, so I hope banks like Natwest reject payments rather than assume entering unarranged overdrafts, particularly for children. However, I am struggling to find anywhere that says they will reject payments like Monzo do except for mentions of an “Unpaid Transaction Fee” which is honestly even worse if that’s just “we will reject direct debits and stuff if you don’t have the money”.