Yes, the Rach Cello Sonata (3rd movement) would probably be the best. Although it is fairly thick writing for Grade 8, it's slow enough to be manageable, and very beautiful and popular too.Good luck,Sam
How about the first movement of Elliott Carter's Cello Sonata No. 1? It's not particularly taxing on either performer and would be entirely accessible to any audience member, and it's approximately 5 minutes long, if not just a tiny bit longer.