Can’t really say because I don’t know much about the Cambridge diet. But i think it really depends on how much milk you are having through the day.
calories add up without you really noticing.
that and a change of activity levels could see less calories being burnt.
if your staying the same weight it could be a lot of reasons other than not losing body fat.
try not to be disheartened I know it’s hard but just stick with your diet plan and the weight will be coming off. Measuring as well along with scales can help you see a better picture. But my favourite is knowing how my clothes are fitting.