I don’t know much about the Newcastle diet. However I have tried various calorie controlled diets over the years and failed to loose enough weight or keep it off. When I was diagnosed Type 2 I adopted a low carb way of eating and have lost over six stone, easily and without getting hungry. I have maintained my blood sugars at non diabetic levels for well over a year. I should add that I take a modest dose of Metformin too.
I have just taken a look at the Slimfast website and each ready made shake is 21.6g carbs, I’m not sure how many you are supposed to have per day, but carbwise two would be my limit.