I came across a dataset this week that had postcodes in this format
Tableau only understands the first half of the postcode, but how do we split it out? Wikipedia as always has a fairly comprehensive description: The outward code (i.e first half) can be from 2 to four characters, but the inward code (second half) is always three characters. Therefore we can isolate the outward code with a calculation:
left([Post Code],len([Post Code])-3)
This now gives the outward code alone in a field that can be given a post code geographic role and used with the filled map mark type.
And if you do spatial analysis, that little anomaly on the river Thames would have caught your attention. Lets add place names and streets in the map layers and zoom in: