VS - I think Skipness and eastern-wiseguy are right. It is unprofessional and discourteous in this context in the UK.
On the other hand, I agree with you. 99.9% of the time I would let it go. But there comes a point where one has been riled so much that you think - why should I? Unfortunately, it is symptomatic of grubby Britain that this is becoming widespread, just like being addressed on first name terms by a vast range of service sectors, just like going into SpecSavers for reading glasses and being spoken to r e a l l y s l o w l y and in LOUD VOICE just because you are over 50.
"Mate" is perfectly acceptable when used by black cab drivers, market traders and as a formal greeting in Australia. In other contexts, I think "sir" or "madam" is polite, neutral, and guaranteed not to cause offence. The problem is that chippy people think they are being forced to be servile - they are not, it is just polite.
Again, the Americans have it right on this one. Say what you like, but they are never less than courteous.