Kiprio UK Postcode
UK postcode lookup with lat/lon, district, ward, constituency
Geocoding, reverse geocoding, places, and static maps.
LatLng API is a free Geocoding API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Complete API documentation for LatLng. Geocoding, reverse geocoding, places search, and vector map tiles.
Details from latlng.work.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
UK postcode lookup with lat/lon, district, ward, constituency
Get the IP geolocation data through the simple REST API. All the responses are JSON encoded
The LatLng API is provided by LatLng Geocoding, Places & Map Tiles API (latlng.work). Complete API documentation for LatLng. Geocoding, reverse geocoding, places search, and vector map tiles.
Yes. The LatLng API is listed in the Geocoding category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The LatLng API requires no authentication, so you can start making requests without signing up.
Yes. The LatLng API supports CORS, so front-end JavaScript can call it directly.
Yes. The LatLng API is served over HTTPS, so it's safe to use in production.