Soup! Incredibly filling, often only £1 for nice fresh ones and usually quite low-cal. Can add bread or other carb if you fond it's a bit lacking in calories and you're very active.
Couscous salad is really easy to assemble - just need hot water and some sliced veg, eg, peppers, tomatoes. You can cook some pastas if they're fresh just by putting them in hot water for 4 mins. Good if you like tortellini.
Also the Innocent veg pots are quite nice and often on offer, also healthy and low cal but I find some a bit bland - need a little salt.