Disenchanted just wanted to point out - by slim-a-soup do you mean those 10 calorie, add boiling water things?
I have had this argument with my mum loads because she does the same thing (reckons because slim-a-soup is only 10 calories and slimfast is circa 200 it'll be better).
I have two concerns:
i) slimfast is set at 200 calories per meal and encouraging of snacks because it keeps your metabolism going throughout the day, so that your body isn't 'shocked' by a 600 cal meal in the evening after slowing down throughout the day.
ii) slimfast has to be made with milk - not water, because without it, it's not a nutritionally complete meal. TBH - ask any nutritionist, and milk is referred to as 'a meal in a glass' because it contains virtually everything your body needs (and doesn't lose anything by being skimmed either ) My aunt used to make slimfast up with water instead, and she became incredibly ill and all her hair fell out/nails broke/skin horrible etc...
It is really important with things like slimfast/lighterlife etc... that you stick to the plan. It's the only way to keep your body steadily in ketosis with no ill effects.
Cheating may help you lose weight faster initially, but it won't be sustainable and your body will suffer...