Covert Affairs is a really baaaaaaaaad spy show starring Piper Perabo, the insipid blonde of Coyote Ugly fame. I think its filming its 4th, maybe 5th season now. If he's guesting on that then I don't want to know about it as its really a crappy formulaic Burn Notice/MI5/Mission Impossible wannabe and he would really be slumming if he made an appearance.
Gotta be very suspicious about Crimson Peak. Stars Tom Hiddleston, Charlie Hunnam and Jessica Chastain, all very hot properties at the moment. He could be doing a couple of days's work, stepping in last minute to a bit part, like he tends to do quite frequently, but there's nothing about it that would call for a contemporary British soldier uniform. Its a Guillermo del Toro film, so it has some prestige to go with it. But, its set near the turn of the 19th Century and is labelled a 'gothic horror romance'. Get this: Hiddleston and Chastain play Sir Thomas and Lady Lucille SHARPE. Take that for what its worth.
Remember is a good candidate, with the storyline "The darkest chapter of the 20th century collides with a contemporary mission of revenge". That one includes some older actors that Sean would probably die to work with: Christopher Plummer, Bruno Ganz, Jurgen Prochnow, Martin Landau. The project is being kept very tightly under wraps with the only clues being: "a thriller about a 90-year-old Holocaust survivor with Alzheimer's who sets out to kill the Nazi war criminal who murdered his family". It has a $13 million budget and world rights sold quickly at Cannes. Another high prestige project.
Pay the Ghost is highly unlikely as its doubtful Cage would ever let Sean share screen time with him after he made him look like such a horse's arse in National Treasure and the story outline doesn't indicate any British soldiers being involved... well, there is a remote possibility that the military could get involved in search and rescue of Cage's character's missing kid.
Pixels is an Adam Sandler-helmed project and also sounds unlikely to have anything to do with the British Army. It includes several Sandler comedy cronies, so unless Sean is venturing into silly Adam Sandler comedy territory, I can't see him putting in an appearance.
There just isn't anything popping up on the radar, anywhere. We'll just have to keep digging, ladies.