Films:
* The Guernsey Literary and Potato Peel Pie Society (2018) as Dawsey Adams
* The King's Speech (2010) as Lionel Logue's son
* The Other Man (2008) as Simon
* The Importance of Being Earnest (2002) as Jack Worthing
* The House of Mirth (2000) as Percy Gryce
Television:
* The Great (2020-present) as Leo Vorontsov
* Victoria (2016-2019) as Lord Melbourne
* The Last Kingdom (2015-2020) as King Alfred
* The Hour (2011-2012) as Freddie Lyon
* Poirot (2004) as David Sheppard
This is not an exhaustive list, but it represents some of his most significant work. He has also appeared in other films and television shows, and his career continues to grow.
You can find a more comprehensive list of his projects on websites like IMDb.