Postal addressing
A postal address is one of the most visible expressions of geographic identity.
Whereas towns can cross county boundaries such that suburbs may lie in a different county to the historic centre, the county in which a single delivery point lies can be definitively identified.
Unfortunately the Standard Royal Mail practice has been to place an invariant county name directly after the town name, thus suggesting that all places within that town are within the same county. This problem is exacerbated by the fact that the Royal Mail's post towns are significantly larger than the towns themselves, potentially bringing many other settlements in adjacent counties under the same umbrella.
As we already know a single delivery point is in a single county. Therefore the solution is to place the county name after the whole address including postcode has been written out, so we do not therefore suggest that the whole post town is part of the same county.
Examples
Newport High Street station High Streeet NEWPORT NP20 4AX Monmouthshire
In this instance, The "NEWPORT" post town and postcode definitively places the location in Monmouthshire.
Where the delivery point is in a suburb of a town which is mostly part of an adjacent county:
Quarry Row Aberbargoed, BARGOED CF81 9AH Monmouthshire
It can be seen in this case that whereas the post town "BARGOED" is mostly in Glamorgan, we are unambiguously stating that our Aberbargoed delivery point is in Monmouthshire — i.e. in this example only "BARGOED CF81 9AH", not the whole of the "BARGOED" post town.
Similarly with:
Wyebank Avenue Tutshill CHEPSTOW NP16 7EJ Gloucestershire
The post town "CHEPSTOW" is mainly in Monmouthshire, but the suburb of Tutshill is in Gloucestershire — i.e. only "CHEPSTOW NP16 7EJ", not the whole of the "CHEPSTOW" post town.
Determining the county for a given postcode
The mapping service at Wikishire allows the user to enter a postcode. It then displays the location of that postcode on a map of the UK and gives the correct county for that delivery point.
Conclusion
Postal addresses in the UK traditionally are written from the smallest address component to the largest — i.e. Recipient → Building → Street → Town → County. Placing the postcode as the last line of the address is therefore illogical as it is smaller than both the county and post town.
Combining the post town and postcode together thus provides a smaller unit than the county and should logically be placed before it. This allows the correct county name to be used unambiguously.