You would have to lose 2lb a week consistently to reach that weight in 16 weeks, and i would have to say that currently at that weight and height i would not consider you to be terribly overweight, although i know of course it depends on build etc, so i don't know how quickly you would lose weight.
All i mean is that WW is about eating healthily and maintaining a lifestyle long term for long term healthy weight loss and maintenance, not crash dieting and losing a lot of weight quickly, which i don't think anyone on here would advocate. Noone can tell you if it would or wouldn't work for you... that's up to you to decide.
I don't think it would be hugely advisable for you to get down to that weight... (bmi of 18.1?!).. because it will be very hard to maintain that weight for life, and your body may just refuse to shift that amount of fat... or not even have that kind of fat to lose...
Someone of your weight will not lose that amount in that time on ANY diet... even a bigger person would still have to work their arse off to shift that weight in that timescale.
I have to agree with Hannata. I think that dropping that much weight is going to leave you under weight. Right now you're in your perfect BMI range for your height, something I would love to get to... I really dont think losing weight should be your focus. I think you should concentrate on toning up. If you work out regularly and tone up from the weight you are now you should lose a few inches and gain some muscle, the more muscle you have the more calories you burn, the healthier you'll be and you wont need to diet.
as you are already within your desired weight range I'm not even sure that Weight Watchers will allow you to join - I can't remember what the requirement is but you have to be more than a certain amount outside your weight range.
I'd certainly be surprised if there were willing to help you get as low as 8 stone 7 as this would put your bmi below 20 and would therefore be seen as having as many detrimental health consequences as being overweight has for those people outside their healthy bmi range. Their philosophy is to get to a healthy weight with the use of a healthy and balanced diet and getting your weight down that low certainly wouldn't fit with that philosophy.
If you really do feel that you need to lose weight then I would suggest you discuss this with your doctor first to hear their view - its possible that you could be putting your health at risk trying to take it too far.
I think at your height and weight you could lose enough weight in 16 weeks to make a significant difference to your size on ww.
I assume you have a reason for stating the low target - have you been that weight before and felt it was the right weight for you?
It does seem low but I think we all have different ideal weights even at the same height. One of my friends is also 5'6'' and she has a fantastic slim figure at 10 st (works out a lot) definitely doesn't need to lose weight. I know when I have been down at 10 st I didn't look as slim as her.
I believe ww will allow you to join if you have 7 lbs to lose so are therefore 7 lbs above a BMI of 20 (I think that is 8 st 12 lbs at 5'6''). So unless the rules have changed you could lose nearly 2 st by going to class. Maybe if you also worked on gaining some muscle too you would be as slim as you want to be but still within the healthy BMI range?
As to whether it will work - I joined ww a few moons ago with a starting weight of 10 st 13 lbs and had no trouble losing weight at a good rate (2lbs/week).