In this instance - the cachers in question live in NZ and the majority of their caches have been found in NZ, they also have a number of hides in NZ
Both have visited the UK, One has found quite a few in the UK so I can see your suggestion working there - but the other has found a few (just over 1% of total finds)
Both come up as being from the UK when it comes to this search
I appreciate the heuristic approach - but that would mean I myself would be constantly changing - This never happened to me however particularly when I spent 3 months in NZ
Anyhow
I would have thought that the logical approach of using the given home location and where the majority of their own cache hides are located as being the country assigned to that cacher in searches like this ?