Hi
Do you know it's definitely milk? Some hospitals have various pre-op diets, mixing milk and l/f yog, some a very low cal/carb diet. I had the option for 4 pints of semi milk a day or unlimited l/f yog, I chose milk, can't alternate days either for mine. You can also generally have no sug jelly, squash and milk shake flavouring. I really like having the milk with a dash of vanilla essence and sweetener. You may also be able to have bovril and stock cubes which give a nice savoury kick in the day.
As far as I know slim fast will be a no no as it does contain sugar and carbs and the pre-op is generally for shrinking the liver so the body using up the glucose or something from this organ, so you shouldn't consume anything with this in.
Hope that was a bit of help?