I was done a few weeks ago for doing 37 in a 30 limit. I had genuinely thought it was a 40 limit, and can remember seeing the camera and thinking I was OK because I was well within the speed limit, so didn't slow down. Even my bloke could've sworn it was a 40 limit, but when we drove that way again, clearly we were wrong.
I therefore can't advise your dad about contesting what speed he was doing. I can, however, tell you what happens next if you don't appeal.
My bloke, as owner of the car, got a letter the same as your dad. We also got some information as to where I could find the photographic evidence on the net (which I'm keeping as a souvenir - I've never been done before either!). He had to send the form back to say he believed it was me (although he knew it was me, but that's what the form says). Then I got a form the same, which I had to send back. I had the option of a £60 fine and 3 points on my licence, or pay £65 for a speed awareness course and not get the points. I chose the course, which I'm doing in a couple of week's time. It's 3 hours on a Tuesday evening. If you live in another county from where you get clocked, they say you can choose a course location closer to your home. It's not run by the police, but by the county council's road safety unit.
My son did a similar course a few years ago. Said it was all very civilised and no-one pointed fingers or made accusations, and he actually learnt quite a bit from it.