Sadly, I think we might indeed be more poncey in the US; you’d (unofficially, ofcourse) be expected to dress formally for an evening performance or event at (say) the Metropolitan Opera House, but the atmosphere and attire is slightly more relaxed for daytime performances/events (business casual). I like the suggested ROH attire better, as it doesn’t require that you get dolled-up just to watch a performance (it’s not like you’re going to be on the bloody stage); you can just relax and enjoy the show.