Can you try this:
curl -v -u 'YOUR_API_KEY:' \
https://api.company-information.service.gov.uk/company/00000006
And if it fails, can you post the 403 body with the key redacted and maybe that might reveal more.
Additionally:
Have a gander at this and see if it’s relevant to you: Issues with creating test company in sandbox API
And this: Reports of invalid auth when using key in test.