the rat could possibly be bain. but it could also being the Dentist. remember, the dentist knows almost everything about the payday crew, and that means he's on the same level of bain, or is bain's accomplice.
The Dentist was also the one to have Hoxton given a plea-deal and therefore transferred. The Dentist got the payday crew to hit the biggest non-federal bank in the country, and got them to hit the largest russian mafia head on the east coast. The Dentist also seems to have political political connection and could explain about hoxton got a transfer and was arrested in the first place.