I second taking your own plate of something
A) it's polite to take something along anyway and
B) It gives you a healthy option you know will be available and that you can fall back on.
Losing weight is also a long term commitment... don't be afraid to enjoy life a little along the way. I'm going to dinner at my sisters tonight for her birthday and I know there will be things there I shouldnt eat. I'll be taking some dips and rice crackers which I know I can have a few of and wont hurt the budget. However i will be indulging in a glass or two of red because hey, its a special occasion and being healthy is for life.
I don't want to be miserable while I'm here!
Have a great time!
Oh and you dont have to have a reason for not wanting to eat something. Just don't - you don't need to explain yourself to anyone.