To be in a 'healthy' bmi range its between 18.5 - 24.9, so at 9 stone your BMI will be 21.6 which is a mid range healthy weight. If you went down to 8 stone your BMI would be 19.2 which is at the lower end of 'healthy'.
So its up to you really if you feel you can get to that weight, which you may well, but it might be maintaining at that weight that may be the hard part, but its how you as a person feel & look.