@Lash,
This isn't America, we don't welcome other countries interfering in our democratic process. Trump has already started, we don't want Sanders doing the same.
Not only that, Corbyn wants to win an election. He's not going to do that with an albatross like Sanders hanging around his neck. Corbyn is the leader of the Labour party, Sanders lost to Hilary Clinton, and now four years after losing he's still not party leader.
I know your system is different, but that will be lost on most British voters, and they're the ones we need to reach. The best response we could get when telling people that Sanders supports Corbyn is "Who the **** is Sanders?"
That's the best response.