I've never understood the point of correcting someone in a way that makes it appear you are trying to make them look stupid. I get a lot of people correcting how I say something as I order, such as the "moe-ka" instead of a "mock-uh" incident mentioned above. If you know what I am ordering, and are aware that my accent is different than yours, why correct the way I say one word? You know what I am ordering, just give it to me with a tad of politeness, please.
The feigned ignorance thing is pointless, too. If you don't know what a person is saying, ask them to explain again. If you are just pretending to not understand because they say it differently than you, it is obvious and makes you look like an a$$, in my opinion. I've seen it here and in the US, and it annoys me in both places.
Another one I hate, and Morrisons in Blackburn seems to be the worst about it, is employees cutting in front of customers, or shoving past them without so much as an excuse me or a 'get out of my way'. It might be different if it was only one rude employee, but it seems to happen at least once per trip, if not with five or six different people. I'd go elsewhere, but Morrisons is the most convenient.