If you must do a liquid type diet, is there any way you could make your own? If you looked at the ingredients list on slimfast and the like, you may not be so keen to use it! It's basically just low fat sugary milkshake with added protein and added vitamins. So even though you wouldn't be malnourished and you would lose weight if you followed it, you would be having a bit of a crappy diet really. I recommend making a yoghurt smoothie for breakfast, a veggie soup with beans in it for lunch, and a veggie soup with a portion of baked fish and salad for dinner. That way, it's a similar set up to the slimfast diet, but you would feel more satisfied, the food would actually taste nice, and you'd still lose the weight quick enough.
Breakfast smoothie could be something like 150g frozen berries (tasty and high in vit C and fibre), 150g low fat greek yoghurt (calcium and protein), 20g oats (fibre and slow release carbs), and 15g of ground almonds (healthy fat and protein). 230 Calories roughly.
Lunch soup (make a bit batch and portion out) carrots, tomatoes, celery, chickpeas/butter beans, spices, onions/leeks, whatever other non starchy veg, boiled together with veg stock and blended. A big bowlful about 200calories.
Dinner, more soup, and 100g salmon or mackerel or whatever white fish you like with some salad and balsamic vinegar. About 500 cals.
Snacks: one apple, one low fat yoghurt, 30g nuts.... total of about 350 cals depending on types.