I'm preparing for the same exam (did DipLCM just over a year ago but been busy with exams etc.) and I plan to play:
Schubert - Sonata in A D.664
Ravel - Sonatine
Ginastera - 3 Danzas Argentinas
I started learning the Ginastera and then found they were on the next diploma up syllabus, so i might change, perhaps. I'm learning rach 23-5 at the moment too, so possibly put that in somewhere, although as you see I've tried to play music from different "eras" as they like to say. I think they also are quite a nice combination. The other possibility would be perhaps some baroque, bach (but I only really like partita 2, italian concerto and french suite no.5) or even rameau - I really like his suite in A and it doesn't look too hard.
I think that the Debussy might seem a bit odd on it's own... although it is in the syllabus, and it is a fantastic piece.
You know, you're allowed 7 mins of whatever you want if it's not on the syllabus. So I think that is worth taking the oppertunity.
I think it's also worth considering something to link all your pieces - whatever you choose will be linked in some way, although it's nice for it to be pretty obvious.
And finally, I think Shostakovich goes nicely with beethoven... i don't know why. But they seem to sound similar, especially when the beethoven sounds humorous...