I must say I thought locust.
Regardless of whether or not you think it’s offensive/inappropriate there are some people who have found it a bad word for the situation. If he had simply said “group of people” or “gathering of people” there’d be no problem. Surely it’s better to use something that’d cause no offence at all to anyone and focus on the actual problem?
I’m with ransos that Cameron probably used this word knowing the connotations it has in order to garner support for sending them back and keeping them out. And Harman saw this word and completely missed the point of her role as leader of the supposedly centre-left opposition.
I’m not usually Mr PC but using unfortunate wording to help breed contempt for vulnerable people is exactly why political correctness now exists. While the average STW member, like cynic-al or scotroutes, is intelligent enough to know what the exact definition of “swarm” is the average member of the public probably isn’t.