I agree that the Special K diet is rubbish- it is SUCH a marketing ploy!! Basically, you could get those xxx number calories (don't know how many in a bowl of the cereal) from something much healthier- such as lean meat, fish, eggs and a salad or fruit- instead of refined corn in the cereal.
Eat sensibly and lose 2lbs a week.
Cut out sugar, white processed flour, fizzy drink, alcohol, fatty and fried foods and live off lean protein, veg and fruit, and some carbs such as wholewheat cereals and bread,- and take some exercise- such as a half hour walk or jog every day.
Slim fast etc is rubbish- it's not healthy and doesn't re-educate your way of eating longer term.