Some do slimfast. My hospital says 3-4 slimfasts a day or 2 slimfasts and a 450 calorie meal.
Mine also offers a very low carb (1 serving a day, a serving being 1 slice of bread or 2 small potatoes etc), low cal diet. 1 carb serving, 3 meat/fish/alternative, 5 veg, 2 fruit and 2 milk. If you want the specific diet ask and I'll try and find a minute to type it out. Or you could look on the BOSPA website cos there is a pre-op diet on there.
Hope this helps x