it was a very irresponsible thing to do if you live in a muslim country, especially a country that is such an active arab league player.
when i was growing up i knew that i had to say "peace be upon him" every time i mentioned the prophet, if i didnt i could cause major offence very easily. saying something bad about the prophet would get you locked up for life no matter what the british embassy tried to say or do. it may seem backward to westerners because saying jesus sucked a donkey's knob is perfectly acceptable, but thats not a good enough reason to ignore foreign culture and assume they will feel the same way, especially when you live there.
end of the day she broke the law in the country she lived in, for that she deserves to be punished in line with local law. imagine if an arab came to the uk and married 6 women, would you say its ok because he isnt familiar with uk laws and customs? it doesnt matter if you know about local laws or if you agree with them, they still apply.