Post Codes
I think a good definition of 'works' is if TomTom takes you more or less to the same point as the latitude+longitude if a Multimap. The buildings associated with any particular post code can be researched using royalmail.co.uk
Unfortunately a recent update from TomTom has created a large number of cases where the input of a post code invites a house number, when no house numbers within the listed set of buildings exists. Instead, TomTom wants to navigate you to another point on the road, where the buildings relate to another post code, often miles away. The only postcodes that work are those relating to genuine streets and houses roads where there are no house numbers, anywhere along the road.
I have reported this by email to TomTom support. The initial response was that I should simply blank out the house number and press 'OK'. However they appear to agree that this does not work and have referred my querry to the next level of technical support.
I have used favorites named in Postcode syle 'TN263AT Ashford Road' using 'My Location' when I actually manage to navigate to a PostCode without the help of TomTom. Trouble is I now find that the number of favorites you can have at any one time is around 50.
When I was in Australia recently, using an expensive download map, the input of an Australian Post Code still invited a house number 3215 Victoria is an area, not a street, so here TomTom have mucked up again.