Lots of people have credit cards to earn airmiles/points/cash back etc on the cards, not because they necessarily need to use (all) the credit available on the card.
Please could you use PSD2 data (whereby customers automatically provide you with information on their spending on their credit cards) to automatically move money each time a customer makes a payment on his/her credit card from that customer’s Monzo current account to a Pot set up to pay off the credit card at the end of the month?
The customer could decide whether 100% of each credit card transaction should be swept into the Pot, or whether some lesser percentage should be swept.
This would have two main benefits for customers: (a) their Monzo current account would more accurately reflect how much available cash (i.e. actual money not credit) they had left to spend that month; and (b) they would be safe in the knowledge they could pay off a certain percentage of their credit card bill at the end of each month.