On the rare occasions that UK police actually have to fire shots at people (who make relatively big, slow-moving targets) they usually do so from fairly close range. Even so,only around 20% of the bullets fired actually hit their targets. To hit a drone flying at several hundred feet you'd probably need to fire several hundred bullets. A .338 bullet (for example) can travel at least two miles, so that's going to be hundreds, or even thousands, of bullets landing on homes, schools, offices and roads near to the airport.
Even if you could manage to position your sharpshooters in such a way that they were only firing into the airport itself, there would still be plenty of aircraft, refuelling facilities, ground crew and passengers in the firing line. (High-powered bullets don't stop when they reach the windows of airport terminals. They go straight through, risking the lives of people inside)