In this situation I'd be more inclined to think UTG is on a flush draw, were he holding an ace he probably would've re-raised on the flop to at least see where he was at, the only other hands I could seem him playing this way would be a medium PP or a weak ace the latter is kinda unlikely since he called from UTG, and the way you played this hand I could see how on the river someone with a medium PP might take a stab at it thinking your flop bet was just a bluff.

I think the flop bet was fine, I'd bet around 150 on the turn and if I'm called I'm fairly certain I'm beat, but I think here is where you stand the best chance of picking up the pot or at least seeing where you are at, on the river you really have no idea if your hand is good or not playing it this way. The way you played it I'd still call the river bet.