The “Pay someone” button on the payments tab has a drop down to make it obvious which account is selected and to switch between them:
However, once you’ve selected a user to pay, the only indication you have of which account is being used is the balance:
This is particularly inconvenient when you click a monzo.me link, because it deeplinks to the “Send Money” screen and if you had the wrong account selected before clicking the link, well… back you go. (Yes, I know the confirmation page says “enter your personal/joint account PIN to confirm” but it’s easy to miss, and it still doesn’t mean I can actually change account if I see it, just restart the journey.)
I think the ideal improvement for me would be to have an account switcher on the “Send Money” page: