Get your ballistics right
Bullets are not magic. They are slowed down by air resistance. using a small, high-velocity round like a Nato 5.56mm or a slower, civilian .22LR, at the kind of steep angle required to shoot at drones (above about 35°) means the round will have shed all its muzzle energy before its reaches the ground again, about four miles away: it will be travelling at its terminal velocity—about as fast as if you'd dropped this tiny piece of jacketed lead from a plane. It might crack a window. It would sting like hell if it hit flesh. The chances of serious damage to thing or person is remote indeed. People in Crawley would be at far greater risk of being run over. (We're not talking about bigger bullets like 7.62mm or even .50-cal: they're just not necessary.)
As for the crippled drone, (a) every airport conducts routine runway sweeps for debris, and clearing up very obvious drone wreckage is hardly a chore compared with an airport shut for 24 hours, and (b) there are actually very few people exposed to the open air around the airport: the drones aren't flying over Horley or Crawley, they're flying—so we're told—over vast swathes of unoccupied runway and threshold.
The props of a drone are extremly vulnerable, and they are fragile. A few rounds through its blades will drop it like a sack of shit. Possibly the worst case is that the drone slews on the way down and hits a grounded plane. But these are not exactly parked wing to wing across the entire airport, they're mostly stuck at the jetways and on the ramps. That calls for some judgment about where to engage the drone; given the huge amount of open field space around Gatwick, this isn't a hard call to make, surely?
The arguments against using small-calibre rifles to shoot them down just do not stack up.