Subway do salads, yes. They're not the best as the meat is so processed - I looked up options when I thought I might get one and I THINK the chicken and bacon salad with ranch sauce is the best one, just ask for mostly green veggies rather than tomatoes and sweetcorn. (Sob, I LOVE SWEETCORN).
You could also take a bag of mini cheeses, or some pork scratchings, or stop by an M+S the first evening and get some meat or prawns to take in?