DT2 7LG - shows Athelhampton Road
DT3 4JG - shows New Barn Road
DT3 4LA - doesn't show any roads, but tapping 'skip' shows a location on the coast, cross referenced, its the same location as Google Maps and its right next to a main road so the favourite will still work just fine.
BH20 5QS - doesn't show any roads, but tapping 'skip' shows a location, I cross referenced this with Google Maps and it shows it as The Lulworth Estate.
Did you get different results to what I posted above? Can you check to be certain you have latest maps (go to Settings > About CoPilot to see the date of the maps, it should have 3.2011 at the beginning).
UPDATE: The above test was with latest shipping iPhone version. I'm retesting on Android device now...