As tempting as it was to just deck Barton right now Bucky didn't make a move, the mission came first. "I was in London with her on mission to find Zola, preferably before he was able to return to Red Skull. We were separated by a suicide bomber and a cave in so she told me to go get help. Natasha spoke highly of you, that's the only reason you're still standing right now, Barton."
Natasha was always one to leave bread crumbs for those that knew where and how to look so that's why he offered to lead. He left out the part where Steve gave the mission, it wasn't his place to put that out there it was Rogers' responsibility to do that.