Why cant we monitor lower airspace for potential intruders more effectively? Like many have pointed out above we face not just low flying aircraft, but now UAVs, or drones or paragliders or whatever.
Is it that difficult? Is it cost against likelihood? Granted we couldn't do the whole UK but how about from the Lizard to the Wash, or Humber even?
Also what do the Israelis do? They must counter their threat somehow.