Hi,
I don't have any experience with anorexia, but I do have first hand experience with depression and anxiety.
First off, don't cause yourself more stress by worrying about your exams. Inform your school and GP of your relapsing and try and inform exam boards/universities as soon as possible. With mitigating circumstances like this, universities like to know preferably before exams, but definitely before they get your results. Also, if things do get too much there are always alternative pathways. Whether you take a few months out, redo A2 (I think you should be able to resit the majority of exams, at least), go through clearing... A2 results are not the be all end all. As someone who left A2 part way through because she couldn't cope, I really respect you for keeping your education a priority through this difficult time and for still working hard. I'm grateful that I could come back and do my A2 exams, but I wish I could have been brave enough to face the year and reach the end.
And that's something good to focus on. Exams will go in the blink of an eye, so at least this stressful time won't last forever. I know it's not a great consolation, but I find it helps.
You do deserve to take medication. NO ONE deserves to suffer, but I know how comforting giving in to your illness and harming yourself can be. Especially at a time like A-Levels where it's nice to feel 'in control' of yourself in some way, but, as you said, despite this 'control', your life isn't improving.
I know all too well the unsettling aspect of rooms, especially the ones you sit exams in. I couldn't even set foot in college without a panic attack this time last year, so I really commend you for battling through. Please don't take me as being condescending. I'm genuinely amazed at anyone who can put up with all of that and still keep going with exams. I don't know if it would help you at all, but I've started wearing ear plugs to exams. I become very hyper aware of sounds when I'm stressed, so they help block noise out and help to 'numb' my mind, if that makes sense. Which helps for exams. Again, please let your school know what's going on and tell them to try and inform exam boards as soon as possible and possibly email universities yourself. Things will only work in your favour if everybody is in the loop, believe me.
Well done on gaining weight. You've probably heard it all before, but recovery really isn't a linear journey. It has major highs and lows. Feeling how you are is in no way a set back. Please share how you're feeling with doctors/family/teachers, anyone who you think will listen and care. Your friends don't sound like very good friends if they're using your illness as an excuse to be downright *****y. I know it's hard to do, but forget about them for the moment. Focus on yourself, your well being and your goals. You'll be in a new situation come September and hopefully in your chosen university, where you can meet new friends. Also, you can get in touch with your chosen universities disability support and tell them how you are, your worries about university and ask about support there. I'm sorry you've experienced such poor care, but perhaps the disability department at your university will be 100x better.
Please feel free to ask me any more questions or just message me if you need somebody to talk to and good luck with the rest of the exam season.