HEY! well i'm preparing for the Dip as well! (Except i'm a US student some i'm still trying to figure out how i'm going to take it)
Many reasons why people do fail the ABSRM is because of the bad quality of playing of pieces that they aren't ready yet. But, I agree with the suggestion of presenting a well rounded complete program of the periods. It's almost as if you were auditioning for a college. If you go to the ABSRM website, they have a complete suggested list the Dip.
I would choose one of the Beethoven Sonatas (COMPLETE) for sure as your classical but I perfer a well-played Op. 14 or Op. 2.
Moxzkowski Etude should be there, but you should include a stronger Romantic piece than the waltz. Maybe a more sophisticated Nocturne instead or a collection of Mazurka's. I question the Impromptu, perhaps one of his other 3 impromptu's would be more suitable, still impressive, and the juries won't be rolling their eyes.
Other Chopin, try Brahms Intermezzo Op. 118 No. 1 and No. 2 or one of Faure Barcolle.
A sinfonia won't cut it, you need to do Prelude and Fugue from WTC or a few movements from the English, French suites. I suggest with the rep your showing me, perhaps D minor, D Major, and G Major from BK. I might be work out.
Also include a Debussy prelude.
Here's what i'm currently learning which might be part of my Diploma program, ( I still have get in contact with ABSRM cause I never taken any exam before)
Bach: Prelude and Fugue in G, WTC I
Schubert: Impromptu Op. 90, No. 2 No. 4
Impromptu Op. 142, No. 4
Scriabin: Selections from Preludes Op. 11, Etude Op. 8 No. 5
Grieg: Sonata Op. 7
Beethoven: 32 Variations in C Minor
Hope this helps.