I agree that the syllabus will not tell me everything; but I am optimistic that my general interest in the subject and 'further reading' would give me some valuable knowledge come exam-time. Also, I think the RE department would be willing to help out if they have some spare time when I come to them with a question. I might be thinking too optimistically, but that's what I'm thinking.
I know that D1-3 are hard to get, and in fact that D1 is a higher grade than an A*, but I'm predicted straight As (2AS, 1 A level). Hopefully this means I have a chance of getting a D1-3, especially as essay subjects are my strongest. Not to mention I decided I wanted to do the Pre-U because the exam questions looked amazing and the mark schemes looked so much better than, say, those of AQA sociology... in the Pre-U you are given some freedom with your answer and you are even credited for extra knowledge. Now that's my kind of exam!
Pages 11 - 19 in the syllabus include some reading, and you can look at past papers and specimen papers you you get an idea of what you need to know and how you need to argue it. There is also plenty of resources on the internet that are well worth using; for example Youtube houses many videos on philosophy.
I want to self-teach philosophy because it wasn't offered as an A level option and it is something I am extremely interested in. How I wanted to do it was basically just read loads of philosophy and then write about it in my blog so I could get feedback on my analytical/reasoning skills. I agree that to learn philosophy you need to learn how to debate it, and that's why I joined the debating club at school. Debating on forums etc would be an option too so I don't think it would be too hard to develop that. A few books on critical thinking have been published too, and you can't forget the Oxford TSA papers as some practise.
I'm sure there are loads of people available for debate, for example I have a Buddhist friend with whom I debate issues of morality and religion. Going on the forum of a political party could also be a way to go; I'm sure a few heated debates could be sparked off there!
Not that it won't be difficult, but I don't think it will be impossible.
I suppose if you don't want to do the exam, you might as well just read as many books on philosophy as you can! May I recommend Nietzsche?
The reason I wanted to do Pre-U in the first place was because the exam looked amazing! XD