gankdat
UK & EU tenders, UK planning, UK sanctions, US exclusions, UK insolvency and incorporations as JSON

UK official public record API.
Gazette Data, UK API is a free Government API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
The Gazette is a rich source of open data that is free for the developer community to use and repurpose unless stated otherwise, is Crown Copyright and is therefore free for you to use under the Open Government Licence.
Details from thegazette.co.uk.

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The Gazette Data, UK API is provided by The Gazette (thegazette.co.uk). The Gazette is a rich source of open data that is free for the developer community to use and repurpose unless stated otherwise, is Crown Copyright and is therefore free for you to use under the Open Government Licence.
Yes. The Gazette Data, UK API is listed in the Government 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.
Yes. The Gazette Data, UK API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the Gazette Data, UK API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Gazette Data, UK API is served over HTTPS, so it's safe to use in production.