They have done before, since its a christmas episode they will probably show it in the christmas season. On another note, since the trade centre incident I havent seen the New York episode, maybe they took it out?
Channel 4 don't show it cos they don't have the rights to season 1 just yet. Also I've heard BBC2 have rights to simpsons until 2006,Maybe channel 4 could get simpsons series 1-10 in 2006 off BBC2? Because BBC2 are definetely NOT gonna show simpsons now.