I think it is do-able.....but we do all lose weight at different rates, so dont aim too high and then you feel upset if you dont quite manage it. Not saying you cant do it,,,,but just think if you just give it 100% and then see how much you lose by the end...you will certainly not be 17 and a half stone that is for sure!
Why is there a need to lose the specific amout for this date? Just curious.
Good luck and honestly, if you give it your all, you will see the weight drop off.