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 (excluding "large user" postcodes). Postcodes are colour coded according to the primary mix of property types within the postcode.
A postcode is only shown as "mixed" use if it contains a significant number of more than one category. Many otherwise residential postcodes contain occasional non-residential properties (eg, electricity substations, schools, etc), while otherwise commercial areas often contain occasional residential properties (eg, caretaker flats). Showing these as mixed use would be misleading, though, as it would make most areas look far more mixed than is actually the case.
You can bookmark this page after zooming in to the location you want, and then return to the bookmarked location afterwards.