It can't be done
Given that -- as I understand it -- most stolen mobile phones end up abroad, this isn't something that can be dealt with on a country-by-country basis. Presumably any code that's sent by a UK network to shut down a handset can be over-ridden by someone reflashing the phone when it arrives in Russia / Africa / insert-bad-country-of-choice.
The only thing I can see that could do the trick would be something that physically damages the circuitry. Maybe phones could all be fitted with small explosive devices which could be activated once the local chavs have got hold of them. Just don't tell the yanks though, they're paranoid enough about what goes onto planes as it is!