So I assume the point you would make if you take the matter to adjudication, is that the sign is badly place since it was moved from the centre traffic island. If the sign had been in its old position, it is very likely you would not have made a U-turn. OK to go with this, but it's not a slam-dunk matter, because the council will emphasise that the sign is there and visible even if you turn left, i.e its not to the right of the T-junction.