I’m travelling to India and Sri Lanka - both these currencies don’t seem to be supported for exchange from GBP through my app - is there a way around this at all/can they be added?
Short answer is no.
Have a look here
transferwise still supports INR balance if you have their card.
you can still use revolut’s card to make purchases and withdrawing INR but they’ll be exchanged on the fly from your base currency… and the weekend surprise can’t be avoided