Diets like slimfast, don't work long term. You better of Finding a plan of attack that you can live with. For me intermittent fasting and low carb, works wonders for me. I have lost 5 stone in 6 months. Also blood sugar are spot on now.
Also when you plan that you cna live with, it makes it easy to add in treats without much impact, Also makes it easy to go back to it after the treat.