I was wondering this today, as I can always spot the British Erasmus students a mile off.
Part of it is use of colour, I think. French women wear lots more black, which hides a lot of sins and slims you down. But they have often got the colouring to get away with it, whereas a paler/pinker-skinned Brit looks washed out in black. So the British wear lots more bright colours, which sometimes work and sometimes don't.
And part of it is also, as has been said, not slavishly following fashion if it doesn't suit you or your age group.
There is also more "casual" clothing in the UK, lots more trainers, sweatshirts and fleeces. Nowt wrong with them per se, but you don't look groomed.
And TBH, though things are changing fast, the French are still on the whole slimmer than the British, so their clothes hang better and don't cling in the wrong places/reveal bulges, etc.