The MidEast to London connection might be a little slow. If you’re willing to spend a bit of extra money, Argo will speed up the connection. I believe it’s $5/month plus some bandwidth costs. You can try it, and if it’s not helping much, turn it back off.
Page Rules can cache the other extensions, but the free plan only gives you three rules. You’d have to Match: example.com/*.apk and set Cache to Cache Everything and then Edge Cache to a month.
It might be possible to manually set cache headers on your site for APK and EPUB, but I don’t know if this will override Cloudflare’s default cached files setting.