Oddly enough, it's American Express... so you're talking some of the biggest spenders in the UK. For our lowest cards you still have to have an individual income of greater than 20k, which rules out the chav working at KFC factor. The vast majority of the customer base is educated, affluent, and high earners.
Not everyone who earns less than £20k a year is a "chav working at KFC".
Your statement suggests that you would expect low earners to be rude or more likely to mock your accent - why? Being on low income doesn't make you a chav or less educated, it just makes you less wealthy. Most low earners and fast food workers would probably not appreciate being looked down upon because they couldn't qualify for an American Express card and lumped together as "chav"
Being a big spender or even "the biggest spenders in the UK" makes you no less "chav" than a someone on benefits on a very limited fixed income, if "chav" is anything at all, it is a lifestyle choice that you can adopt at any income level, Kerry Katona, Katie Price, Wayne Rooney are all "chav" ( in my opinion), and they could certainly qualify for any credit card going.
I used to work and train people in a bank call centre, for collections actually, and very rarely had a bad experience, and these were desperate people about to lose their houses, often they'd start off angry and shouty, but by the end they'd be sorry and crying, when people are scared or frustrated or angry they'll lash out, and as a disembodied voice on a phone, you're the obvious target. It's part of the job and the downside of call centres the world over, you either have to learn a way of not letting it get to you, or of finding somewhere else to work (even in this harsh economy)
I'm a real believer that when you're hitting a wall with something, the easiest change you can make is within yourself, you can't change your customers or easily change your job, but your attitude and your outlook is easily changed with a little effort, and in doing so you might find that you're able to deal with things much more easily without letting them upset you. Yes it's a pain and unfair that you have to do that, but you'll feel better quicker, trust me!