I've lost a best friend and it's more than hard. Your circumstances sound so much worse than mine though. All I'd like to do is give you a big hug.
How long is it since your friend died? It does get better with time, I still think of my friend pretty much every day but it does get better, it doesn't control my life it it shouldn't control yours. Stay strong, if you are ready talk to you parents or someone you trust. Or maybe try ringing Cruise or some other bereavement helpline, I've heard they're good (it could also possible be some sort of practise for court, just to get you used to talking about it). But if you aren't ready to talk that's fine too, it takes time.
Having to give evidence in court sounds awful, a huge worry. But I'm pretty sure you will feel better after it's over. Try not to think so negatively about it. Your freind wouldn't want you to be unhappy on account of her.
Sometime you will have to face what has happened. And it's not going to be perfect and happy after that but thats life. I have no idea what you believe but if you do ever get the chance to meet her again you may as well have some good stories to tell her. Keep living.
I don't know much about eating disorders but I do know I wouldn't have eaten for a few weeks after had my parents not been giving me food. I certainly didn't feel much like eating and I'm quite happy with my body (well, more of a don't really care anymore). I don't blame you for getting an ed. Maybe try looking someplace other than the NHS for help? I don't know of anywhere but a google might just come up with something. Good luck.