I was told there was some conversation at the Nottingham community meetup about unique card issuing for different circumstances with the current account.
I see @tom mentioned ideas about this here: Idea to solve the problem of changing card number on loads of subscriptions
As browsers move forward to support the Web Payment API, it would be interesting if you were to able to issue unique cards per merchant and the browser could segment the card limit on that PAN to the payment cost. Monzo actually holds enough payment info here that the browser could prefill based on all the data available in your system.
Further discussion of the idea here on a Mozilla developers payment api repo.