Solved! Go to Solution.
Currently, you can only geocode using the first half of the postal code. In the December release of ArcGIS Online, you'll find that you will be able to geocode the entire postal code in the UK.
*EDIT* If you have access to ArcMap: *EDIT*
Another way in the meantime would be to get the OS Code-Point Open data (it's free) from http://www.ordnancesurvey.co.uk/oswebsite/products/code-point-open/index.html
You get a point feature for each full postcode in the UK, simply join the data in your .txt or .csv file to it based on the postcode, selecting to only keep matching features, and your data is geocoded to full postcode level.
You might need to play around with the codes as the OS data doesn't have a space between the incode and outcode where the first number has two digits, e.g. LS4 2DD and LS153EF. I've found the best way to do this is to create new fields for incode and outcode and use the field calculator to separate them before recombining them according to how your data is formatted.
Hope that helps...
Dan