The geocaches has been assigned to counties, by polygon definitions we have. We then "guess" which region a county belongs to by statistics. Ie, if county X has 90% of the geocaches in region Y, it's likely to be inside that one.
The map you linked isn't a drawing of the region in itself, but all the counties inside it, filled with white or green.
IF the "sum" of a couple of counties actually equals the region definitions, we could in theory join the county polygons into regions. There are two issues here.
* We can't be sure the borders for the regions actually are the same. A county could exist over regions, especially since we don't know where the region definitions are from.
* Joining several polygons into one works very well if the share borders completely. That often isn't the case and there are then gaps in the middle of them. An example could be that a river between two counties. In those cases the county polygon is most likely defined wrong, but it's a fairly common case. The join region polygon will then look quite crap.
To be honest, we haven't tried this for UK. Since we don't do it anywhere else, we don't really have the tools to investigate it and see the end result. In short, it's quite a lot of work, and we don't know if the end result will be good enough to keep.
I hope I explained it thorough enough.