Leaving aside the question of Hybrids, Ray Clarkes record I belive was 17.5 inches to tail fork...perhaps Ray is about to confirm that...
Assuming it was measured correctly, a Roach of 16-17 inches to tail fork -not to tail tip- should be a 3 pound fish. As this fish is very fat it should easily be a 3lb fish.
However !
I know you said no decomposition but my initial reacation on seeing the photo was that this was a fish that was already dead and had started to take on water so bloating it.
I have a feeling it may weigh less than it looks.