So, within the UK using the BACS and FPS network any transfers have to be GBP (from my understanding). If you were to send money from an account in another currency, it would have to be send via SWIFT which Monzo doesn’t support (they don’t have an IBAN). They are looking at doing Transferwise however: International transfer
So, I think you’d still have to get Payonner to pay you in GDP with their 2% charge. 