I'm 5'7, and I know my ideal weight range according to the NHS BMI calculator is anything between 8st 6lbs and 11.5lbs. 8st 6lb, for me, is ridiculously light, unattainably so, and at 11st 5lbs I'm probably about half a stone heavier than I feel I should be as I start looking a bit heavy and feel a bit jiggly. I've got a 'medium' frame, (check how much your wrist circumference is to get a rough idea), and I'm naturally quite muscular; defined abs, even without working out, big biceps and solid calves and thighs, etc. For me, about 10st 5lbs is my 'ideal' weight.
You're 5'6 and a size 10-12: I'd say that sounds pretty perfect, although according to BMI you're probably a few pounds 'over' the ideal.
I wonder if you might suffer with water retention issues? You say you have 'naturally terrible' legs, so am wondering if that means cellulite? If so, then cut out tea, coffee, ciggies, alcohol, and eat more parsley and green herbs. A good sauna can see you lose about 5lbs, if it's just carrying too much water....