I saw it yesterday. I liked it and was surprised at how many people were in the theater. Without spoiling anything, part of the story was about the conflict between the Downton Abbey household staff and that of the royal family, which had precedence. That was also a plot element of Gosford Park, a film from a while back that was also written by Julian Fellowes.
And I have an etiquette question that even those who didn’t see the movie might be able to address. The trailer makes it clear that the story kicks off with Robert Crawley receiving a letter that the king and queen are to visit Downton Abbey and stay overnight (based, by the way, on a real trip taken by the real-life royals to Yorkshire at this time). It sounds a bit like they’re inviting themselves. That sounds improper. We don’t see the text of the letter, but perhaps it’s phrased delicately, like “We are coming to your area and should like to stay at your estate, if you’ll have us.” And of course he’s not going to say no.
And BTW, this New York Time article (paywall warning) goes into the history of royal visits. They were a big deal but very expensive for the host; perhaps even ruinously so.