When I moved to the UK in '97, I drove here in my car from Germany. No probs at all. However, the insurance was horrendous as it was a non-UK car (steering wheel on the left hand side).
The "EU" sign you're referring to has probably to do with the 'newer' number plates, that means if you change residences a lot, you have to register your car each time anew at the local Vehicle + Transport Dept. (in Germany the number plates specify the town/area you're from, I think the same goes for Austria), they take the old plates off and pin new ones on. If your Dad didn't move in the past 10-12 years then his car will probably still have its original number plates (without the blue vertical stripe on the left [?] side). I don't think this is a concern.