In looking a little closer at Britton's velocity - it doesn't appear that it's been down this year at all, nor has it been inconsistent. Here is a snap shot of his velo last year as compared to this year:
But for the record, I also looked at a couple isolated starts this year, and his velocity was consistent game to game, so I'm really not seeing any variance here. Perhaps the stadium guns were off a bit, not sure, but according to Pitch F/X, he's been pretty consistent with his velo.
I think that with Britton he's just been inconsistent with commanding the sinker, and overall fastball command. When it's not sinking, and the fastball is up, he gets crushed. Also, too many damn walks.