Take a look at the bit on top gear last week charting the progress of a letter posted on ios, s-61 from ios - penzance, road via truro for sorting to exeter, then EXT-EMA flight and on from there.
If you can still get it its a good watch, shows the network of road and aircraft that royal mail use, just really baffled me as to why they moved it by road to EXT and then upto EMA by air instead of straight NQY-EMA perhaps using skybus or wow as a contractor to carry the mail and parcels ?