I've just discovered something that might be useful to some people.
When I set up my Apple TV originally, I added my iCloud account then my US iTunes account and then my UK iTunes account. All fine. However, when I went to use Siri to search for shows specifically on platforms like HBO Now or Showtime it would be hit and miss. For example, it could never find Ray Donovan or Dexter on Showtime and it could never find John Oliver or The Pacific on HBO. I also couldn't tell it to search for shows "on Hulu" or "on HBO" because it'd say it can't do that in my region.
Well, today I was fiddle farting around with it as I normally do, and for one reason or another I decided to log out of all my iTunes accounts on the Apple TV, which in effect deleted them from the device. Once they were all gone, I then chose United States as my region and rebooted the Apple TV.
Once it had rebooted, I went back into settings, but this time the first iTunes account I added was my US iTunes account. Doing this fixed Siri right up. It now searches Netflix, HBO, Hulu and Showtime with no issues at all, no matter what I throw at it. Afterwards, I went and added my Australian and UK accounts, but switched back to the US one after they were added, and Siri continues to work as if I am in the US.
Don't worry, it still understands my accent without a problem.
TL;DR Delete all your iTunes accounts, switch region to US, reboot, add your US iTunes account first, then add other accounts afterwards and Siri will work just as if you're in America.