Meltema
Multi-model weather: GFS, ECMWF AIFS/IFS and a 31-member GEFS ensemble, keyless point forecasts
UK weather forecasts and climate data.
Met Office DataHub API is a free Weather API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.

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Yes. The Met Office DataHub API is listed in the Weather 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 Met Office DataHub API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Met Office DataHub API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Met Office DataHub API is served over HTTPS, so it's safe to use in production.