Most pubs or general restaurants will allow you to make changes to your plate, swapping chips for a jacket, etc.
I went to Fankie & Benny's last week and had the maple glazed gammon with 2 pineapple rings, a jacket and a side of green veggies. It would have been 4.5 Syns in total, but I used butter on the jacket because it was a bit dry, but still 10.5 Syns wasn't bad at all! And it was lovely!!
Like Ali said, Harvesters have the free salad bar, which is always a fantastic idea!