I do not think you are only eating 600 calories a day. You say your biscuits are 200 calories, a bag of crisps (depending on which ones you eat) are a minimum of 100 calories the chocolate bar is another minimum 100 calories. Your evening meal of chicken with veg: Only 200 calories?! Does that include potatoes? How are they cooked? Roasted is high in calories, mashed (with butter) will be high, how do you cook the chicken? Fried? High in cals, with the skin on again high in cals. I think if you took a proper look you are probably eating far more calories than you realise.