My lunch is usually 2 slices of granary / whole grain bread, with extra light philly cheese instead of butter, some wafer thin ham and salad.
A couple of other potential lunches depending on facilities and taste
Pasta salad - use low/non fat cottage cheese, low fat caeser sauce (Cardini I think it's called), lf cream fraiche or vinagrette. Could add in a hard boiled egg and salad.
Roast vegetable couscous - Ainsley Harriot packs, other flavours too, just add boiling water and stir it. Goes great with a small tin of tuna stirred through it and some veg (could microwave some peas and sweetcorn).
Or if you can microwave at work then you could make up some light pasta with some tuna, veg and a LF tomato sauce and then just reheat. Same goes for rice eg risotto etc.