I see nothing wrong with shorts and T-shirts if they are in good condition and clean although generally this is not how I travel. Judging by the reactions to the OP I wonder if I was the only one who got the implication.
"
Sitting there having a meal, when in comes the blokes, with singlets, shorts and thongs and start to down their bourbon and cokes."
I thought the OP was being economical with his words and I got quite a different picture than most responses who see the OP as making a statement about social class and others financial status. I don't see it that way, I think it is clear he is talking about their cringe-worthy behaviour. They could all be bankers but it is the behaviour that categorises them. But then I grew up reading My Brother Jack.