Well, admittedly, many Americans misappropriate usages of the term "microaggression" at times.

In my view, when your coworkers excluded you from group lunch-making, that WOULDN'T be a microaggression if it was based on the actual knowledge that you individually prefer to bring your own lunch. However, if they are making the assumption that you'd bring your own lunch BECAUSE you're from Yorkshire, that would be a microaggression.

Making assumptions about people based on their accent (absent any other knowledge about the person) generally is a microaggression, because you're linking behavior or reputation to a characteristic over which they have very limited control. If you noticed someone's accent and inquired what part of a country they were originally from, that in and of itself WOULDN'T be a microaggression -- because asking that question isn't inherently making any value-judgments (positive or negative) about them.

Regarding the protected classes insofar as codified law...that's all well and good, that your government states it wants/intends to protect them. How that actually goes, in practice, within schools (or, more specifically, within the classrooms of individual educators) is a whole other matter. Power-abusers tend to ignore pesky things like laws when it comes to attempts at abusing their power.