How rude some of you are. Surely you could be less aggressive to a newbie and just point him in the right direction. Why reply if your just going to give grief.
Nadhirah, I hope you are showing some initiative of your own and emailing the different airlines at LHR. If it's the ops/engineering/ATC related I think it will be quite hard due to security clearances etc (I once tried BA Engineering and they rejected my request for work experience)
How ever I know RAF North Holt and London City airport offer limited work experience places for students during the year. You could try them.
London City Airport : Work experience
RAF Northolt - Visiting RAF Northolt
Good luck mate