Why Britain is fat:
vanity sizing
the fashion for lycra, low waistbands, pjs, tracksuits and shell suits (or any super-comfortable clothing that just expands as your waist grows)
fructose added to everything as a cheap but calorific sweetener
high consumption of soft drinks
low fat foods (they can tempt people to eat more of it, and it's often full of sugar and salt)
high fat foods in excess - especially takeaways, which many people now eat several times a week
Less exercise - higher car ownership, computer games etc, remote controls, central heating - it all contributes to less energy being consumed
New norms - if more people are overweight, it's easier to think you don't have a problem
I suggest:
Put up very large mirrors all around shopping centres in supermarkets etc so people can see what they look like when they walk, shop, and eat. I am not joking either.