Those who quote security considerations, maybe should look what has been done at some airports and military airfields, in that those who come just to watch the aircraft are encouraged to be eyes and ears for the police.
Heathrow does in fact operate such a scheme:
Metropolitan Police Service - Heathrow - Aviation Enthusiast Scheme
LCY also has a scheme where members are encouraged to report "subjects videoing or photographing aircraft or making notes of flight times"
