It may be an idea to apply to your nearest airport (LGW?), where you'll probably end up following different people each day at a different bit of the airport. for example security one day, ATC the next, etc.....
That all depends on their security regulations though, if your nearest is LGW then BAA might not offer work experience!
But i reccomend applying to an airline based at the airport or even an airline that has a maintenance hangar or an engineering company at the airport. It would be interesting to see how one of the big carriers is run/structured or helping with the maintenance of their aircraft.
I've applied for work experience at a place near me called "ATC Lasham" who do overhauls, refits, maintenance, safety checks, etc... for many big operators. From what i have gathered so far, i will be right up-close to the aircraft and most likely assisting an engineer!