Normally I have English (US) on top in my Settings>General>Language & Region>Preferred Languages.
Do you think this is what Apple means by U.S. English? I guess this might be interesting for all that have English as their mother language and want to test the new Siri. For me, being from Norway it is even more interesting since it might be years before Norwegian is supported.
I think your “region” must also be the US though, and that has other effects which you might not want, only some of which can be undone with other settings (I don’t, for instance, believe you can choose a spelling dictionary separately). That wouldn’t be a worthwhile compromise in my case. Fortunately I believe the UK isn’t too far behind, so I’ll have it by the end of the year.