My guess is TransferWise captured the UK<> Poland corridor and expanded from there.
This problem is a great one to solve because the customers have a real pain and one can be very targeted about capturing high-value corridors.
I wish TW the best. Lower fee, Lower friction, Lesser time == Great for customers!
Anyone looking to build a startup in this domain should start here: http://www.worldbank.org/en/topic/migrationremittancesdiaspo...
(This data is useful to understand origin<>destination of remittance flows; Its a greater than $150B market and a lot of untapped/Uncaptured areas; lots still left to do!)