As an IDVT (identity document validation technology) provider, part of the process is a facial similarity check. This checks the photo on your ID against a live selfie that you provide.
When using a birth certificate and proof of national insurance, we are unable to complete this part of the check so the result will be "consider".
Note that if you have a UK DBS check as part of your request, you will be prompted to complete an identity check using a valid photo ID and this is to remain compliant with the DBS guidelines.
In general, a birth certificate and proof of national insurance can be used to prove your right to work in the UK as a natural-born UK citizen. If you do not have a valid photo ID and have a DBS check as part of your request, your employer will need to review your birth certificate in person in order to proceed with the DBS.