Original post by JunoWell, stop and think about what you're asking. You're expecting someone to put on an exam for you months after the GCSE period! They'll have to create a new exam paper just for you - everyone else who needs it will have already resat, but they can't give you the same paper in case you cheat (either intentionally or accidentally). They also can't use your paper again in case you share it. Then they'll need special markers to mark just your paper, and a special way of transporting it securely, not to mention the fact that you'll need your own exam centre and invigilators etc.
There are loads of ways to resit GCSE English (and maths, actually). Some colleges don't do any other GCSEs but will let people take those two. You just have to do it when the exam boards say, and follow their rules.
I would strongly recommend taking lessons. Part of any GCSE - or any exam - is not just knowing the subject in general, but knowing the specific things that you will be tested on. I'm in the last year of my degree and recently I had to complete a level 2 (same level as GCSE) course for work, and although in general I found it easy there were bits I didn't know.
It is up to you, but if you fail you'll need another year out.
Yes, you can ask your previous school. They may not allow external candidates, but you need to find out. Prices vary between exam centres, so check out local colleges and other schools to see if you can find a better deal. Some people on TSR have said they paid as low as £25 per paper, but others report being charged £200+.
What subject do you want to do at uni? Can you do something related to that? You could work and save money for uni, and then see if you could volunteer in your spare time.