Slimming World are pretty good. I joined last year and lost nearly a stone in 6 weeks, but then I went on holiday and other things came up and I couldn't go to the meetings.
They're pricey (abotu £10 to join then £4.75, I think, for each session, and you have to pay for any sessions you miss), but you can buy the starter pack and books on e-bay so you can do it yourself at home.
Most fruit and veg are free foods so you can eat as much as you like, meat is free on a red day but limited on a green day, and you have things like bread, dairy etc as healthy extras. Chocolate, biscuits etc have a syn value (listed in the starter pack) and you can have up to 15 syns per day. For eg a bag of crisps is 5 syns, I think, and a smear of butter is 3, or something like that.
It is very easy to follow, but TBH you can do just as well stuffing yourself on fruit and veg, limiting, but not removing, your wheat and dairy intake, and allowing yourself the occasional choccy treat.
Actually, having typed this, I'm going to start following it again and see if I can shift a bit more flab.
HTH.