I played the 1st movement of Ravel's violin sonata for my main piece at AS level. It's on the LRSM syllabus so it's fairly advanced, though I reckon having someone really good on the piano part (as I did!) makes it much easier and more enjoyable to play.
For my 2nd performance I played Rachmaninov's B major prelude for piano. It's about grade 6 standard.
For my A2 coursework, I played the piano and recorded Ravel's Pavane pour une infante defunte (an easy grade 8) and Bach's Italian Concerto Mvmt 3 (a more tricky grade 8)... I sent one of them off. For my recital I played lots of 20th century violin music (Ravel, Hindemith, Berkeley, Martinu and some jazz), some easy stuff and some more technically demanding pieces.
I was equally confident with everything I played and got decent A grades for my performances.