I am neither a teacher nor a professional, but please allow me to give my humble opinions.
If your K310 is c.20 minutes, that means you are going to play repeats. I wouldn't suggest so. If you cut all the repeats, then you will have spare time for you to put up a romantic piece of 6-7 minutes.
BTW, you don't need to play in the order of periods. Some people suggest a candidate should play the one he/she is most confident first, while the last piece should have a grand ending. I passed my LTCL exam last year (100% by self-learning), and my order was Mozart -> Prokofiev -> Bach -> Ravel (4 pieces only, each last about 10 minutes).
Good luck.