I'd guess most of them, if you actually consider the whole world.
Probably not enough to matter, if you are only looking at the UK.
Oh, good point. I forgot about China.
Actually, alternative app stores can make use of that as well. The installer app needs to be installed as a system app with proper permissions, and it will them be able to background install apps on demand just like Google play does. The F-Droid Privileged Extension[0] does just that. While you need to be rooted to add/register such apps, that's not a problem for device manufacturers.
As of Oreo Google doesn't allow this for devices if they're to pass CTS without users jumping through multiple hoops first.
If you open an APK it prompts you to go right to Settings and enable it. Actually easier than installing Firefox on a brand new Windows Surface, which requires an MS account to enable unknown app sources.