I re-read this thread last night and I think perhaps I (and maybe others?) may have missed the key poInt of the OP. I don't think it was obfuscating, but I think reading quickly, as many of us do, I missed the representation issue as I was thinking 'secret Santa?!'
I spent a year in a Muslim school in Riyadh and we were taught that any representation (image, play, films) is against the teachings of Islam. I have since been told that that was a very literal and fundamental interpretation of the teachings, and that other Muslims believe differently and take a more liberal view.
Now, if after that level of immersion (three hours daily of Koran study, in Arabic) I am still misunderstanding and being corrected by Muslim friends, then I can see very easily why this thread has stirred such high feelings.
I also observe that the British are extremely emotional about Christmas; it's a time of year for dropping the stiff upper lip and feeling warm and sentimental and inclusive. Hence the 3rd lobster shorthand, because lots of us see the modern nativity as a more inclusive and non-specific play that is more about celebrating hope and love than attempting a specific Christian dictate.
I think the OP perhaps needs to clarify the question? Are you being unreasonable not wanting your child to take part in something specifically and absolutely forbidden by his religion? I don't think anyone would think that was unreasonable at all.
Do people think that involvement as a nonprophet in a nativity play us haram? That could be a question for Islamic scholars or, as one of my friends suggested, a very personal question only you and your family should answer.
The integration issue is a whole other thread!!
I think ypu dud the right thing to post and open the debate; there is too much entrenched belief around these issues and if we, as parents and aquaintances, can keep a dialogue going, however heated, that can only be a good thing.
Anyway, whatever you choose, I hope that you enjoy the holidays and this exciting time of year!