The postcode map only shows currently active postcodes. You need to zoom in to locality level to see postcode markers, in order to prevent the server (and your browser) from crying.
This map shows all currently active normal geographic postcodes containing residential properties. It doesn't include "large user" residential postcodes, such as those assigned to university halls of residence or care homes. Postcodes are colour coded according to the median council tax band within them.
You can bookmark this page after zooming in to the location you want, and then return to the bookmarked location afterwards.