Nadine Keels’s review published on Letterboxd:
Everything it needs to be, it is.
And yes, you do have to already love Downton to love this as you should. The continuity from the television series to the film is well done, both where the plot and the characters are concerned, and it's upbeat and satisfying overall without making every detail of every character's life perfect.
It was great hearing all the music again, too. Downton's theme, the Mary/Matthew theme, the Tom/Sybil theme...
Whenever I rewatch the series now, I'll be sure to include this in the cycle.