Is it possible to top up with a debit card from outside UK…perhaps selecting an amount in Euro with Monzo converting that to a GBP value
I don’t think so. You would need to transfer to a UK bank and then transfer out.
EDIT: I stand corrected
You can top up with a foreign debit card, but your bank may charge you (extortionate) fees 
I too up with an American card and it always works