Lily James posted a pic on Istagram of her, Laura Carmichael, Allen Leech and an actor named Douglas Booh.
http://instagram.com/p/rZSHVSlTxf/?modal=true
I don't know if he is the guy playing Rose's fiance or if he was just hanging around with LC, LJ and AL. It could be him, but it's difficult to tell... also he has a stubble so unless he finished filming a few days ago, it's not him.