Currently leverages 4 APIs to validate postcodes for the United Kingdom:

Postcode Validation
Convenience method to validate a postcode. Returns true or false (meaning valid or invalid respectively)

Postcode Autocomplete
Convenience method to return a list of matching postcodes.

Postcode Info Lookup
This uniquely identifies a postcode.
Returns a single postcode entity for a given postcode (case, space insensitive).

Reverse Geocoding
Returns nearest postcodes for a given longitude and latitude.