I studied in Edinburgh, started 1st year when I was 17, nearly 18. I could have gone a year earlier, as I'd passed my Highers and got unconditional offers to my choices of university, but I stayed on for 6th year at school as I felt that I wouldn't be ready to study at almost 17. It was nothing to do with legal drinking ages, I'd been going to pubs for 2yrs by then, I just felt that I'd be more mature a year down the line. And I was.
I also spent a very profitable year working for 2 A-levels that I didn't need, with no pressure, so I could read around the subjects and really explore them, and I also took an extra language O-grade. It was a great year, and I grew much more independent, open-minded and savvy during the course of it.
The French school system is, IME, pretty infantilisant, and if he is to take real advantage of his high-profile Southern English university, he might be better off taking a gap year of some kind. When I was an undergraduate, the rich public school kids who'd just returned from a year-long trip to Guatemala were by no means more intelligent than the other students - on any level - but they were extremely arrogant and condescending confident. It takes a degree of confidence in oneself to prevent those types from occupying more space than they deserve in a university context.