It's a costume party.
I'd give him license to dress up how he likes.
A muslim terrorist, a KKK in a white sheet, or even a lawyer.
People dress up in humorous, absurd, or controversial ways at a costume party. It doesn't entail advocating anything in particular.
How anyone can pretend it does is beyond me.
It's a costume party!
Dear humorless people of England, get a bloody life.
And if he would have dressed as Arafat, OBL, or Saddam, or even blew up innocents in a suicide belt, he would have been cast as a victim of "occupation" by the US and Israel.