Howdy!
I had a thought based on some new functionality that Capital One has in the US.
Essentially, they have a new app where you can generate one off card numbers that are tied to your account. You can then use those numbers with online merchants so that if that number is compromised, it is useless, as it could only be used once. (Very similar idea to Apply Pay and Google Pay tokens).
The interesting addition they have is that you can optionally create long lived card numbers, that becomes tied to that merchant on first use. So you could create a card number to be used exclusively on Amazon, one to be used exclusively on Paypal, etc. That way you don’t need to constantly enter a new number, but it also becomes useless if its ever discovered and used elsewhere.
This kind of functionality sounds like it would be amazing on Monzo.
Cheers!