I think 4 stone by Christmas might be pushing it, I think 3 and a bit might be doable at your weight and you will still see a massive difference in your shape and how you feel at that weight loss.
The biggest thing with weight loss is realising and accepting your part in how you got to be that weight in the first place and accepting that you won't be able to lose weight and go back to that way of life if you want to keep the weight off. People say diets don't work, which of course they don't if you don't actually do them. You need to totally change your mindset when it comes to food and drink.
I do think exercise is important, out bodies are designed for it, but nothing too drastic at this point or you will injure yourself, people of your and my weight tend to have more muscle anyway as we need it to carry around the excess weight, so doing things to maintain that muscle in the bigger muscles in your thighs and bum is something to concentrate on. Exercise that is low impact that improves your range of movement is good too, so yoga and pilates, rediscover your core ! Helps with posture which helps with looking better at any weight 
Exercise doesn't need to be something you have to go somewhere to do, if you are anything like me, I can always think of a million and one excuses not to go to a gym. If you have stairs you have the equivalent of a step machine, you can pick up weights cheaply enough and there are lots of videos online you can do, even walking from room to room, do it so you bring your knee's up as high as you can on each step. Do squats whilst waiting for something in the microwave or the kettle to boil, all these things add up over the course of a day and you don't need to leave your house or put on special clothing. Don't think 'I can't exercise', think 'how can I squeeze in some exercise into this moment' !
And don't be disillusioned by the plateau, it happens, its normal, keep going.