This is a hypothesis about how variation relates to the period of time in which a language has been in an area. Simply put, the more diversity within a region, the more likely that place is the origin [1].
So in this case, England will have the most linguistic diversity within English since it originated there.
This hypothesis has an analogue within population genetics, e.g. Africa has the most genetic diversity compared to other regions.
I don't know if this effect has been given a name yet, but it would seem to apply to any system where 1) mutations accumulate over time 2) populations tend to be stable but 3) groups occasionally move about very large distances.
[1] https://www.reddit.com/r/AskAnthropology/comments/3dq1ko/why...
[2] https://www.sciencedirect.com/science/article/pii/S096098220...