Use CustomBroadcastArea to estimate phones in bleed area

Our current assumption is that the bleed area has the same population
density as the broadcast area.

This is particularly naïve when:
- the bleed area overlaps the sea – no-one lives in the sea
- the broadcast area is a village and the bleed area is the surrounding
  countryside
- the broadcast area is adjacent to a densely populated area like a city

We can be smarter about this now that we have a way of determining the
number of phones in an arbitrary area, based on the known areas that we
have population data about.

Calculating the population in an overlap is a slightly more intensive
calculation. So we only doing it for areas which are smaller enough that
it doesn’t slow things down too much. For larger areas we still use the
more naïve algorithm.

app/models/broadcast_message.py

@@ -7,6 +7,7 @@ from orderedset import OrderedSet
 from werkzeug.utils import cached_property
 
 from app.broadcast_areas.models import (
+    CustomBroadcastArea,
     CustomBroadcastAreas,
     broadcast_area_libraries,
 )
@@ -17,6 +18,10 @@ from app.notify_client.broadcast_message_api_client import (
     broadcast_message_api_client,
 )
 
+ESTIMATED_AREA_OF_LARGEST_UK_COUNTY = broadcast_area_libraries.get_areas(
+    'ctyua19-E10000023'  # North Yorkshire
+)[0].polygons.estimated_area
+
 
 class BroadcastMessage(JSONModel):
 
@@ -176,12 +181,18 @@ class BroadcastMessage(JSONModel):
 
     @property
     def count_of_phones_likely(self):
-        area_estimate = self.simple_polygons.estimated_area
-        bleed_area_estimate = self.simple_polygons_with_bleed.estimated_area - area_estimate
-        return round_to_significant_figures(
-            self.count_of_phones + (self.count_of_phones * bleed_area_estimate / area_estimate),
-            1
-        )
+        estimated_area = self.simple_polygons.estimated_area
+
+        if estimated_area > ESTIMATED_AREA_OF_LARGEST_UK_COUNTY:
+            count = self.count_of_phones * (
+                self.simple_polygons_with_bleed.estimated_area / estimated_area
+            )
+        else:
+            count = CustomBroadcastArea.from_polygon_objects(
+                self.simple_polygons_with_bleed
+            ).count_of_phones
+
+        return round_to_significant_figures(count, 1)
 
     def get_areas(self, areas):
         return broadcast_area_libraries.get_areas(