There is a limit to what governments can do, unless you're going to ban crisps, sweets, cakes & squash/sodas completely. I mean, it would be NICE if the sugar content of all sold foods was limited to 15%, squash, aspartame-containing foods, sodas and hydrogenated fats were banned, but ain't gonna happen.
Get the TV out of your house if you want to limit the influence of advertising, is my No. 1 tip.
Besides, if you make these foods too forbidden the kids will just go to great lengths to get them, anyway. The only thing I completely ban my kids from having is chewing gum (Blech!).
Don't buy them, don't offer to them, don't have in the house, junk food. Emphasise if they get a cake or sweet that it's a treat, something special, not an everyday thing, not a staple in their diet. I tell my kids they need "balance" and lots of biscuits is not "balance".
Sadly, I think most people have pretty bad diets and don't really mind their kids having likewise. Few parents actually get 5 portions of fruit+veg in themselves each day, never mind into the kids. I seem to be the only mom who dislikes the kids having squash at Mom+Tots groups & Nursery. DH has a cousin who is a parenting perfectionist in many respects, but the only cereal in her cupboard less than 35% sugar was ReadyBrek. When I complained on Mumsnet that Party Bags shouldn't be chock full of chocolate & sweets I got pooh-poohed by the other Mumsnetters. Yesterday I saw a 5yo drinking her own, 2/3 litre size bottle of Dr. Pepper. 2yos eating crisps for breakfast in every pushchair in town, 7 month old's contentedly working thru chocolate bars ("My Health Visitor said it's ok!" is the reply when I ask).
A friend had toddlers grow cress for a special event. She warned us about an E.Coli risk if our kids actually ate them, "But it doesn't matter because they are never going to eat them, anyway", she laughed. Well, my 3yo gobbled them down.
-Z (Food Nazi Extraordinaire?)