It's much less to do with the content of the ad itself than the process by which stuff gets pulled. The Advertising Standards Authority looks into every complaint, and if it's upheld, then an ad gets pulled.
So it's got nothing to do with popular opinion really – if ONE person complains and the ASA thinks they're right, out it goes. In my experience though (ad industry for 7 years now) it rarely gets that far – as soon as a brand gets a whiff that there might be a backlash against an ad, they pull it themselves. Because it's cheaper to lose the money you spent making the ad and buying the space to show it than it is to be on the front of the daily mail three days in a row because you're 'glamourising knife crime.' Stupid, narrow-minded, unrepresentative, undemocratic, but that's how these things work.