Dont want to make assumptions, but with your starting weight, if they are available in your area you may be eligible for Slimming World on referral vouchers - I started at just under 13 stone, and 5ft4 with a BMI of just over 30 which entitled me to free vouchers. It means free membership and up to 6 months of classes, saving me about £130 - I've just used up my allocation and am now converting to a paying member.
Just be aware, whether its available at all, or how many free classes you get, varies by local authority/primary care trust etc. (and lets not get into that whole debate again of whether or not its fair - just trying to make people aware that it might be an option)