Doubleblade is right. Although I did MEP for O levels almost 20 years ago, I don't think the criteria has changed much (but I may be wrong). One section requires you to have ABRSM equievalent grades in practical (one instrument at least) and theory. In my time, the higher grade you have, the more marks you score for that section. I had Grade 8 for theory, Grade 8 for piano and Grade 3 for trumpet at that time - so I got quite high marks even before I sit for the exam. One of my friends had a diploma in violin, Grade 8 in theory, and grade 8 in flute. Needless to say, he also got distinction.
The most important written exam requires a lot of music analysis on selected pieces. I believe I had about 6 pieces we had to study and analyse in detail including 2 symphonies, a few tone poems, and a few others I can't remember (vaguely remembered Till Eulenspiegel, Hebrides overture, a Schubert and a Mozart symphony, can't remember the rest). We need to know all the different eras all the way up to 20th-century music and their pecularities (we were also asked to compose music but not examinable - the best one is when we had to compose 20th-century stuff).
The last part is aural exam which is really quite easy. Simple rhythm, melody writing and intervals recognition according to what the examiner played on the piano. About Grade 4-5 level.
Oh, in my time, there is no fixed textbook. We do have a number of scores we had to buy and highlight all over the place to analyse them. Scores are not cheap - luckily the school bought it in bulk. But we also had to get the music (cassette tapes in my time - CD nowadays). My MEP teacher used to tell us, "Music is not a cheap hobby." He's absolutely right.
I learned a lot through MEP. The music analysis part is not something you can learn from ABRSM exams. Definitely helped me a lot in my foundational years of music writing.
And yes, most of us get distinctions.
