You have no choice. Even with skim the calorie count is huge at 401 per serving. You simply cannot afford full fat. I suggest you use nut milk instead. Unsweetened almond milk is 30 calories a cup (240 ml) and at fewer than 1g carbs, it is far lower in carbs than any dairy milk.
Having said that, the ingredients of the Optislim shake are not BSD friendly, in my view. There are multiple sugars and starches. It’s not just the mere calorie count you have to worry about. The source of them matters more than the final figure because not all calories are equal. If you have blood sugar issues, these ingredients could work against you. Being on a liquid diet will likely help you drop weight anyway, though.
Best of luck to you!