Do you liked Downton Abbey? How about well-researched historical fiction? How about gay romance set against the backdrop of World War I? How do you feel about gross sobbing? I know I did it more than a few times reading it.
I give you the Halo Effect series by [Unknown LJ tag]
Part 1. Halo Effect - When Thomas rescues Lady Mary from the nefarious attentions of Turkish houseguest Kemal Pamuk, his employers and coworkers begin to see him differently...and Things Get Better. A Season One AU diverging from canon in Episode 3, The One Where The Turkish Gentleman Dies in Mary's Bed.
Part 2 - Soldier's Heart - Following the events of "Halo Effect," Thomas Barrow goes to war. A season two AU.
I hear the author plans a season three in the long term, but is currently working on a one-shot set in WWII where Thomas and his boyfriend take in an evacuee from the blitz.