Postcodes.io

Postcodes.io provides a free API for validating UK postcodes, retrieving detailed geographic, administrative and demographic data for a given postcode, and performing bulk reverse‑geocoding of latitude/longitude coordinates to their nearest postcode. It also offers endpoints to find nearby postcodes and look up outward code information.

curl -X GET \
  "https://api.postcodes.io/postcodes/SW1A1AA"