Have a look on Autotrader to see what cars similar to yours are going for and get it listed tomorrow. Hopefully a private sale will get you an extra few hundred quid. She's already said she doesn't want it so make sure you get the keys and hide them. I'd also be tempted to move the car and park it somewhere else (just incase she wanted to spite you by damaging it). Is there anyone who would let you park it on their drive or outside their house?
Once it's sold and you have that cash you can make moves to get out of there. In the mean time, try your absolute best to save every penny you can.
Also, make sure you gather up anything sentimental, valuable and important documents. She sounds like a horrid woman and it wouldn't surprise me if she broke something of yours that you cared about, or hid things like your passport. If you have a friend that can mind them for you, I'd box them all up and drop them off asap.
I would also, personally, not tell her that you're moving out. Keep it quiet until you are literally standing in your new place. She might react badly and kick you out now, knowing you don't have anywhere to go. Or, she might go the other way and start guilt tripping you, manipulating you into staying for her own selfish needs. If she's still being hateful, you don't need to tell her your new address. I'd also make sure that you change the address on things like bank statements, etc, before you tell her you've moved out. The last thing you need is her rifling through your post or refusing to hand it over to you.
You can absolutely do this. But, as hard as it is, you need to be calm and have a plan. The last thing you need is her finding out, reacting badly, and sabotaging you.