My teacher wants me to go in for my grade six piano. However, I have only ever done grade one, so my theory is somewhat inferior to my playing. I want to then carry on to do my grade seven and eight. I am in year 12 now and doing all three would help me acquire more UCAS points. How long do you think it would take me to learn the theory, and do you think I would be able to do at least grade six and seven before I leave school?
You won't need grade VI theory. Most boards expect grade V theory to be passed before a candidate takes grade VI, VII and VIII practical - on any instrument.
The theory will help with your playing - to a degree. No, not a degree exam! You need to concentrate on reading the syllabus for grade V theory, and within say 6 mths, be ready for the exam. That happens three times a year, and teacher will know when. The practical exams take about a year each for an average player - whatever that is!
Roughly speaking, grade 5 music theory is considered equivalent to a UK GCSE exam. So it's not a trivial thing to do.
There are theory books, and exercise books to buy. You can buy past exam papers from the ABRSM, along with the answers to go with them. So it's possible to test yourself before taking the exam.