I can't speak for Spain but I don't really agree about France. When I lived there, people certainly did go out to get pissed and did drive despite knowing they were pissed.Yes, I think this ties in with the nature of drinking previously mentioned, where Brits deliberately set out to get drunk. If you´re gonna get "proper pissed", driving is out of the question. If you go out in Spain/France and end up having one or two drinks more than you expected with food, you can probably convince yourself you´re not "really" too drunk to drive.
I remember being berated by everyone I was with for refusing a lift from a colleague after a night out. When I said he shouldn't be driving because he was drunk and I certainly wasn't getting in a car with him, they pointed out I was drunk too. And when I said yes but I was getting the metro home and not getting behind the wheel of a car, they all thought I was crazy. I saw people do this quite often. It was very normal.
And this despite a work colleague having crashed a few years before when driving while pissed, killing himself and seriously injuring another couple of colleagues.
This was Paris btw so had excellent public transport. I've heard it's even worse in the country.
This idea that the French are sophisticated and drink responsibly doesn't really match my experience .