Hey Will!
It's great that you are so curious about what you can do in the future, it means you are very conscientious and take a keen interest in your own personal development!
There are lots of threads on here about the things you want to know, I can summarise a bit of what I know for you if you like....
1. After Grade 8 on any instrument there are lots of choices you can make, and you can be as flexible as you like. You can do a DipABRSM (which I think is a lot tougher than Grade 8 so you will take a while to prepare) in your instrument, and with either a performance, teaching or direction pathway. The ABRSM has syllabuses you can ask for and take a look at....
2. In terms of theory, you can go all the way up to Grade 8 if you like, and after that the LCM does Theory diplomas at various advanced levels (go to
http://mercury.tvu.ac.uk/lcmexams/)3. Composition is more difficult, you could study it at university or music college, or take a Diploma from the LCM like I am studying to do... I don't know if anyone else knows anything better about composition diplomas? As far as I know it is the only one available. They do graded exams in composition too although they are very different from anything else I have seen, but very thorough! A friend of mine studied for Composition at the LCM for a degree, and she loved it
4. Hmmmm arranging.... I'd be interested in that myself if anyone knows of any places I could check? I studied Orchestration at University which helped, as did my composition skills. You will find you will want to arrange things once you start composing anyway I think, or at least I did, it's a great skill to develop especially if you want to teach. You could always pop to your local music library if you have one, and pick up a few simpe and interesting looking scores to arrange?
Hope this helps! All of the ABRSM Theory and Practical exams count towards UCAS points too now I think...