I've been watching the under 21s since December ish and Brannagan is a player I like. Not as commanding and passing not as expansive as Rossiter, not as creative as Teixeira, but a player who likes to keep busy in the middle, demands the ball, defensively okay, good work rate and has good composure for a kid. He can play as a #10 or in central midfield.
Personally, I don't think he's as stand out as Rossiter and Teixeira, but he has all the key attributes to become a very good player. But saying that, It doesn't matter if you have all the potential in the world. If they don't get the opportunity and don't have the determination and desire then it's pretty much pointless.
Anyway, I think it would be disappointed if we sold him without a buy back clause but I would rather us loan him out. I personally think we'll miss him for the u21s, I'd rather he stayed and given a chance to prove his worth as he's been with us since, 8 or 9 years of age?