Its not designed to be used like that. If you are intent on doing this then please make sure you take a multivitamin and mineral to replace the ones you are not getting from slimfast.
I know you want the weight off but in another post you said you feel deprived if you can't have food, deprivation is in my opinion the biggest diet breaker...800 cals will see the weight come off quick but how will you cope without food. I think your original plan of 2 shakes and a meal would be a better option.
I'm not having a pop at you but i'd hate to see you fall off the wagon because you're feeling deprived. Have you tried the 3-2-1 plan and failed? If not give it a go before going so extreme. This is a long tough journey that doesn't need to be made any tougher by being so strict.
This isn't sustainable. There are too many carbs in slimfast to use as a VLCD therefore you won't go into ketosis. You will therefore be starving your body. If you want a diet like that try one of the many VLCD. If not use slimfast with food. The weight does come off. With a VLCD you can not vary at all not a morsel of food. With slimfast if you go out for lunch or cant get a shake you can juggle things around to keep in your calories.