National theater at home available?

Sometimes services are location restricted, and that often happens with British content. However, I think your problem was you were not spelling theatre the same way they do. When I searched the channel store for National Theater, I found nothing. But searching for National Theatre, I found this:

https://channelstore.roku.com/details/971f5455f07936a2e03fb467f2e7a78b/national-theatre-at-home