edit to make clear which part i was responding to!:
So any Mastercard used abroad can benefit as do Monzo, Revolut and
Starling?
Unfortunately not - most banks choose to add a fee on top of mastercards exchange rate - some specialist cards (e.g. Halifax clarity) and the new banks (Monzo, Starling, Revolut) are not adding fees on.
I believe the banks actually have to pay some fee themselves to mastercard for foreign transactions - the banks above are currently absorbing this themselves. Its much lower on card payment than it is for ATM withdrawal (which Tom has mentioned a few times as being expensive for the running of Monzo and that they would prefer if people use the card abroad primarily for payment)