Airport police in the UK carry H&K MP5s, with a selector for single-round or three-round-burst fire only - no fully automatic rock'n'roll. The reason is that a semi-auto carbine with sights is much more accurate than a pistol (especially if shooting across a crowded airport concourse), and rifle ammunition is out of the question (to easy to penetrate multiple partition walls and kill bystanders). So, pistol ammunition but a more accurate gun.

As for what they're expecting? This is a genuine Scottish tabloid newspaper cover from a couple of years back:

http://current.com/entertainment/wtf/88862229_i-kicked-burni...

There have been attacks on British airports in recent memory: not often, but often enough to justify the permanent armed police presence. And then there are the security nightmares everyone in the security biz remembers, such as the Lod Airport massacre:

http://en.wikipedia.org/wiki/Lod_Airport_massacre