Spoilers ahead...
I was an avid watcher of the series. But where it ended with hope that the family had awoken to the realities of the world they live in, this movie sets them back to the same stuffy attitudes as they were in the first season. Not to mention an un-interesting plot and contrived progression - ”OMG, the king and queen are coming, make sure the silverware is polished - it's not, bring Carson out of retirement to take over”.
In the end the Downton staff get the privilege and realize their life long dream of cooking and serving for royalty - and are hardly noticed. Waste of good actors.