I include some county information with my UK postcode download data, but I get quite a few questions regarding it. The most common question is why don’t all the postcodes have an associated county. The answer is that county information is only shown for postcodes that are located in an administrative county council. So LA1 postcodes are listed as part of Lancashire but BB1 postcodes aren’t, even though most people would consider Blackburn to be in Lancashire. This page gives you an idea of how this works. Select ‘Counties’ in the dropdown and see all the gaps.
The second question that generally follows is whether I could include county information for each postcode. This is where things get tricky. Have a read of this Wikipedia page on the subject of counties. In short, there are the administrative county councils in use today, there are historical counties whose boundaries have changed many times and there are postal counties that used to be supplied by the Royal Mail.
So if I wanted to add county information for every postcode, my first decision would be which of these to use. The Royal Mail seem pretty keen to get rid of postal counties and the information is not provided with the freely available postcode data, so that’s not an option.
So another option would be to use historical counties. Leaving aside the fact these boundaries changed many times, I’m not sure using them would satisfy most users of my download data anyway. Taking an example from the referenced Wikipedia page, how many people would consider Brixton to be in Surrey?
And the final option would be to fill the county data from the local authority. So Brixton would now be in London, but BB1 would now be in ‘Blackburn with Darwen’. Now in this case it’s obvious that any postcodes in the ‘Blackburn with Darwen’ area should show Lancashire as the county so I could possibly map all these authorities to sensible counties, with some work. But even then, would this provide people with what they want? Kingston would be in London, although some people would expect it to be in Surrey, since Surrey county council is based there (and use Surrey in their address).
So in conclusion, counties are blooming tricky and I suspect no one size would fit all, hence the incomplete set of data on my site. If anyone has an opinion on whether there is a good approach to this, let me know.