Taz is right, if you aim to lose 1.5lbs each week that would give you a total loss over 14 weeks of 1.5 stone. It's perfectly achievable to lose more, but i find when i'm on a losing roll it usually averages out at 1.5lbs a week. Plus, if you lose more it'll be a huge bonus!
The success express plan is supposed to shift the pounds a bit quicker, which could be worth a try if you wanted to get things moving quicker.