In reply to the OP I watched it this morning (after it taking a few hours to appear on the iPlayer). I'm not actually in the least bit surprised that so few on the teams knew what Kosher was. I think Jenny and Michael did sort of really know but got very wrapped up in the task far away from the rest of the team and started to believe in their own importance and what they wanted to think must be right. This got magnified with the other task they were given to buy! There were several others in that team who were looking for chickens but seemed to have only a very vague idea, what would have happened had they had to buy one? I don't think many people have probably ever heard the word Kosher.
As for the saying Michael wasn't Jewish, couldn't understand that myself, if one of his parents is Jewish I can't see how he isn't ethnically Jewish and saying so is bang out of order really - unless he lied about that?