I still have a few Cambridge bars from my ill-fated venture into that particular territory. Am pleased with how I am doing on SW - but would it be ok, now and again, to have a Cambridge bar instead of a SW- friendly meal?
The point of following and paying for a particular plan is that you follow THAT plan. Of course there's no harm in eating a meal replacement bar but it's missing the point a bit. SW teaches you to eat a balanced diet and also enjoy a wide range of foods. It is very much designed around home cooking, recipes, flavour, filling meals etc. The Cambridge diet I don't know much about but given it uses meal replacement bars I would guess it's very different.
Personally, to do well at SW you have to do SW. I would avoid going against the "ethos" of SW and just try your best to follow the plan. Obviously the world will not implode if you were to eat a bar but your week might not be as good perhaps!