I'm not doing slimfast so feel free to disregard but I'd say if you are that hungry then have something, an extra 100-200 calories to stop you going off on a mad one...the lesser of 2 evils. either have half a shake and make it hot or cook something low cal. I've done slimfast years ago and I didnt feel like i've eaten if everything I had was cold, it was the only time i struggled.
Same goes now, if i eat a sandwich or a salad with cold meat, and have another cold meal later it just doesnt work, i dont feel satisfied...a huge bowl of green veg with half a tablespoon of olive oil and some herbs and spices does the trick for me...all for around 100 cals