12 year old dd overweight! I am joining the gym with her in jan (which will be a struggle) but she won't eat anything healthy!

[Welshcakesareyum]

My dd is very over weight, it has crept on over the last few years. She has always been a fussy eater. I am joining the gym with her in jan but she won't eat anything healthy at all!
Do I tell her it's that (healthy option) or nothing at all. Or a bowl of cereal?
All she will eat is:
Pizza
Chicken wraps
Sandwiches/crisps
Yorkshire pudding/chicken/gravy
Cereal
Toast
Bread
Anything sweet

Don't they say that weight is something like 80% food choices, and only 20% exercise?
Don't get me wrong, generally moving and hopefully finding a sport or exercise she enjoys is a positive thing - although I'm not sure 'the gym' is often the most appealing to a child (?) - but you need to really sort the eating.
Difficult to know how to suggest you go about this as I'm not sure how she has got to 12 without eating any fruit, veg, and not a lot of protein (and I speak as Mum to one fussy one {out of 3 dc} so I know it isn't as simple as some think it is when you get a fussy one.

Does she have any sensory issues?
What do you think would be a reaction to going 'cold turkey' with the sweet stuff ? After all, eating sugars makes you crave sugars - if she could break the cycle, then would that work for her ?
Does she want things to change? (which will obviously make things easier)
Would it work if you said you wanted to try a month without 'sweet' things, and just not have them in the house ?
Does she have access to money / shops / choices at school when she can just get her fix there if you go down that route?
Would she be happier being involved in choices ?

Does she cook ? If she has made a meal herself it can be quite a motivator.

Nothing really I can add to that. I think Back has covered everything I was going to say