Wow, you have every right to be angry with your parents for this. Are they getting any help for their alcoholism? If not, perhaps you should get in touch with social services.
Congratulations on your Oxford offer. They obviously believe in you, so you must do your best to get there! Now, your home situation has been going on for a long time, and you should be focusing on yourself and your future. When I was studying for my A-levels, I used to stay at college until 6 o'clock most nights to work in the library, and my dad used to pick me up on his way home from work. I only did that because of all the extra resources available in the library, and because I had no MSN or siblings to distract me there, but it still made a massive difference. Another thing I can suggest is that you talk to your friends. Don't go into loads of detail, just say to a friend you trust that you're having problems at home, and would it be ok if you revise together at their house? I used to do this with a couple of my friends, and as long as you're not likely to distract each other it can work really well. You can sit there companionably revising, and if you need a break, you can test each other on what you know.
As for university funding, if you go to Oxford you'll be taking a gap year, so I suggest you move out and rent a room somewhere cheap, perhaps in a student house. Then you can get a job and save up money to help you through university. It will be hard, but I'm sure you can do it. Good luck!
PS - if you want any English-specific revision tips, PM me and I'll give you some ideas.