Slimfast isn't that high in protein, although they are better than nothing.
Getting Whey Protein powder can give you 20-30g of protein in one drink, a bit more if you mix them with milk. But they do cost a fair bit and some taste like c**p!
I've tried a few and my favs are BSN Desert Protein Powder (internet about £18.00 a tub). I don't have them very often, only on days where I know I've not eaten enough.
Some teams don't recommend having any protein powders.,They do recommend fortified milk (adding skimmed milk powder to your normal milk).
Hope it gives you some ideas.