Illya moves at almost the same time she does, and he sees the weapon a second before it fires. Not at him, and the girl has magic. Smart to display it? No. But she may have no better way to defend herself. He, however, does.

Which is why he unzips his jacket as he turns toward the attacker. His hand pulls out the pistol he has. Two quick shots send blood spattering and the man falling. After which he puts the gun back in its holster and covers it with his jacket. Just because guns aren't supposed to be carried around England doesn't mean he's just giving his up.

Then, he looks over the girl, assessing to see if she's been further harmed.