You will lose weight on slimfast, but it's a liquid diet, so you'd lose weight on any liquid diet. People tend to regain the weight they lost when they stop the diet. Although Slimfast now make other meal supplements like ready meals, soups and cereal bars.
If you want to try a liquid diet, why don't you try juicing. You can use both vegetables and fruits to make nutritious juice drinks. You could have a juice for breakfast and one for lunch and a balanced, healthy meal in the evening.
Most stores, which sell electricals sell a variety of juicing machines which also come with a recipe book, so you can make lots of different tasting juices. Infact, my partner bought me one today for my birthday, think it's a Breville professional one. I can't wait to use it.
Apparently Jordan swears by juicing too and lost all her baby weight using this method.