The symbols are characters in Aurebesh, the Star Wars alphabet. Aurebesh is based on simple character substitution, so it's easy to tell you that those are the characters for "UK."
Less easy is what that actually means. Best I can tell, this is a kind of identifier for the United Kingdom branch of the 501st Legion, an organization of enthusiastic cosplayers. I've found this discussed on one of the club's forums, where you can see the decal being applied to members' armour (click to embiggen):
As far as I know, no members of the 501st have appeared in armour in any Star Wars film. If forced to guess, I would assume that whoever composited that image borrowed a publicity shot from the UK Garrison, possibly without realizing that it wasn't an official image; film-accurate costumes is one of the 501st's goals.
This decal seems to be highly non-standard, even among other branches of the 501st; based on the discussion in that forum thread I linked to, the 501st generally disallows "non-canon" markings at formal appearances. UKG appears to be an exception, though I'm not familiar enough with the club's internal politics to speculate on why.