"In fact this has already happens in session circles where people who play English tunes feel invaded by Irish music and get rather snooty about it." I haven't found that - I suppose we tend to generalise from our own experience.

I think that there are probably English features about the way English musicians play other types of music (such as American or Irish) which are discernable to musicians from the other traditions.

The other thing is, regional differences are still important within England - eg up in the North-East it's really another country.