[boundaries] Using state again for UK countries (England, Scotland, Wales, Northern Ireland). country_region was created mostly for non-administrative regions of a country (usually admin_level=3 in OSM). The UK is a bit more complicated in that there are multiple non-sovereign countries, but it's probably not worth creating a different tag and different set of parameters just to have a distinct name for 1st level admin in the UK

resources/boundaries/osm/gb.yaml

@@ -1,8 +1,8 @@
 ---
     admin_level: 
         "2": "country"
-        "4": "country_region"
-        "5": "state"
+        "4": "state"
+        "5": "state_district"
         "6": "state_district"
         "8": "city"
         "10": "suburb"