If Bob
slowed down to 40 on this specific section of dual carriageway, he must have been zipping past the drivers who will have slowed down to 30..
imo, Bob should accept these three points without a fight, and then transfer the V5 to his father, who will then automatically get any further speeding fines.. If Bob decides to deny he was driving his own car, then the police may get a little suspicious..
I also wouldn't trust the quality of any photograph the police decide to let you see, they may just have a better resolution one they are saving for a court appearance..