The climate might have something to do with it. You burn more calories if you're cold, and from what I've seen people in England use central heating far more than in France in the winter (and it actually get a lots colder here in the south of France than in the south of England). Most people I know round where I live heat their houses to no more than 16 or 17° In the winter, where as my English friends seem to have far warmer houses (22 or 23°).
I work in a school, and out of 500 kids, only 3 are obese, and a small handful are visibly overweight (maybe 5 or 6). We live in quite a poor town on a river, in spring, summer and autumn lots of families walk down there and spend the day having fun and moving about a lot, there is also a communal swimming pool which I end up avoiding as there are so many pupils there from morning until it closes. The kids from richer families in the surrounding villages seem to all have pools, so they spend a lot of the summer using lots of energy too.
As its already been said, kids here don't snack, they have four meals a day (breakfast before 8, lunch at 12, goûter at 4 and dinner at 8), the majority have school meals which are very healthy.
There are plenty of McDo's, and quite often people go there for a special treat
, a couple of times a month at the most. In the supermarkets there is plenty of crap, but far far fewer special offers, and individual packets of crisps are very very limited (you can buy a pack of 6 individual packets, either all ready salted, or two chicken, two ready salted and two BBQ ).
And yes, people very rarely eat on the move. Lycéens (15 to 18yos) quite often go out and grab a sandwich (une Americaine = half a baguette with a burger and chips in it) but they'll then go and find somewhere to sit down to eat it with their friends.