I am very aware that if someone is "different", they stand out and thus, attract more scrutiny. I've just been to a dance competition, so this issue is on my mind. In the last two days, I've watched hundreds of girls dance in groups. If one girl is heavier than all the others, or has super-skinny legs, or her costume is slightly different from the others, or has short hair when the others have a bun, the eye naturally goes to that girl. It is human nature. So to some degree, the success of the team depends more on how that "different" one dances. If the different one dances well, that is great. If the different one makes a mistake, however, it is more likely to be noticed than the mistakes of the other girls.
I don't think we can or should train ourselves to not notice people who are "different" ... it is really a survival thing that is built into us (e.g. from an evolutionary standpoint, we are programmed to notice when a member of the enemy tribe tries to slip into our territory. The Crime Watch folks still tell us to look for someone who is "out of place" in our neighborhoods.) Also, if someone is acting differently, they may be ill or otherwise in need of help.
So, how does that relate to Brits at WDW? We Americans notice the accent because it is different (most of us love to listen to it). Hearing an accent makes us pay a bit more attention to those persons than we pay to the thousands of other people at the parks. So, if you have a different accent and your behavior is exemplary, it is noted. However, if your children are screaming at the top of their lungs or are otherwise behaving badly, that is noticed also. At WDW, everyone sort of tunes out the voices of whiney kids and cross parents. But, if the voice we hear attracts our attention in some way (e.g. a British accent), then we pay more attention to those whines and cross words than we do to all of the others. We might hear 50 obnoxious American kids and 1 obnoxious British kid, but we're more likely to notice the obnoxious British kid.
And of course, the Brits get more scrutiny, because your accent attracts our attention AND we can understand what you are saying. If someone speaking Chinese is saying something rude, I'll never know it.
It happens to me with other American tourists all the time. I'll cringe when I overhear the insensitive things that other Americans say in Europe. But the Japanese guy in the same gift shop might be saying something just as bad, but I'll never know it. Thus, I get the impression of my fellow Americans being the rude ones.