I would like to know if I can add postcode information onto the already existing names and lat,longs - I am building a very simple mobile app - and adding the postcode would make my life much easier.
You can look into utilizing google's maps. It supports reverse geocoding and will give you the full address including zipcode for any lat/lng. It is free and easy: http://www.gps-data-team.com/map/
OS Open Data Code point is the UK source of postcode data and is free to download and use for both personal and commercial use.
You could use this to get the northing and easting of every postcode and then combine it with a northing to easting convertor to get the longitude and latitude for the postcode.
You could use the postcode data as a reference point behind your app.
Most of the northing/easting to longitude/latitude convertors are written in either JavaScript or PHP. You could either use them to change the postcodes in a batch or if you feel comfortable you could rewrite the convertor in the language of your mobile app.