As was mentioned already - 0.5-1kg a week loss is a good aim. If you lose weight too fast you run the risks of
a) putting it all back on once you start eating "normally"
b) ending up with loose flabby skin because you didn't tone it up!
So I'd recommend doing lots of exercise to accompany your weight loss, and to try maybe spend half the time on one of these shake programs (never heard of the one you mentioned, but I know OptiFast is recommended by a lot of doctors, you can buy it at the chemist) to kick start your loss, while exercising as well, and then spend the second half of your holidays concentrating on making healthy eating habits that will last a lifetime.. you don't want to lose 20kg every summer only to put it back on during the year!
I wish you the best of luck, its certainly possible to lose 10-15kg in the 3 months if you put heaps of effort in.. you will definitely notice the difference, and will be at goal soon after!