BartK take a look at the Clementi sonatas on the RCM grade 8 syllabus. They seemed to be on the longer side. I haven’t listened to them so I’m not sure how appealing they are… my experience with Clementi is limited to a couple of the sonatinas, which were good for me as a student but I wouldn’t go out of my way to play them. You could also look at Beethoven Opus 2 no 1. This is RCM grade 10, which I think is pretty comparable to ABRSM grade 8. It’s early Beethoven, but I think it’s a fantastic piece, and I have a soft spot for it, as it was one of the pieces I worked on in high school. I think RCM has a couple other Beethoven sonatas at this level in their grade 10 syllabus (although they test on 2 movements, not the full sonata). Another piece you could consider is the Mozart c minor fantasie, which is 10 pages. It’s a beautiful piece, but I think one of the benefits of a sonata is that you have the faster outer movements/slower inner movement, and you don’t get this in the Fantasie.
The Peer Gynt suite sounds like a good option.