Original post by Alleykat606Pick a nice baroque/classical piece. Look at Bach's Preludes and Fugues both WTC I and II, Mozart Sonatas, Haydn Sonatas, Beethoven Sonatas. Remember you'll have to play the complete sonata, not just a single movement (though you probably know that already!)
I'd then pick a big romantic piece; look at pieces by Schumann, Chopin, Liszt and Brahms.
With the sonatine, that should cover the time allocation but if it doesn't, I'm guessing you're only going to need probably 5 more minutes of playing so choose a relaxing, delicate, modern piece (can also be your own choice i.e not on the set list) perhaps Gershwin, Schoenberg, Scriabin, Shostakovitch, Stravinsky. Since you've already chosen the Ravel, I'd stay clear of another impressionist/French/poetic piece so don't bother looking at Faure, Poulenc, Debussy etc.
Regards to your question about 'practice' pieces - use the finger exercises by Dohnanyi; I cannot stress this enough - these exercises should be your bible - just simply set aside 5-10mins of your practice time for these exercises, they really really help, trust me! For actual 'pieces' I'd suggest starting with some Chopin Etudes both Op. 10 and 25. I started with op 10 no 5 and op 25 no 12 a couple of years ago, I'd say those are the two that are most manageable. For now at DipABRSM level, stay well clear of all Liszt studies, Rachmaninoff studies, Scriabin studies, Saint-Saens etc. etc. Chopin's an excellent starting place for etudes yet still gives you enough of a technical challenge!
If you need any more advice, feel free to ask - I've played a lot of what's on the diploma set lists, including the Sonatine, so ask away!