You've lost 12 stone 8 in weight? that's an amazing achievement! that's like my entire body weight!, well done!
My advice would be;
You can't outrun a bad diet - learn to eat well, (minimal junk, minimal processed food) we all know how easy it is to slip into a spiral, and become part of the majority of people who put all the weight back on again 2 years later.
Try to find something you enjoy for exercise - many people start off and give up after 6 months, it's all about finding something you like that you'll stick with, I've been doing fitness classes for about 9 years straight because of the social aspect and fun factor, my other friend is a cycling addict, we've both stuck to it because we find it fun, others enjoy weight training, I used to be a competitive swimmer - but it bores me to tears now. I think Nuffield gyms are pretty decent - they offer a lot of stuff that you can try out, don't be afraid to book into different things, boxercise is great fun - as are the les mills classes, circuit training is also very good - and all the nuffield gyms have lots of these, it might half kill you the first few times, but stick with it and your fitness will get good quickly.
Learn to do it forever, as part of your life, my dad has been a lifelong runner - he had to stop and switch to cycling when he hit 75 because of his back, but he's very fit - and it's the ultimate pension payout, as aside from being caught out by bad luck - he easily has many more years ahead of him, because he made an effort to keep fit for all those years.
