Is there nobody else at your dad's co who can sign a reference? I had the same issue with my son, but had somebody else at the company write the reference for him. As an authorised signatory, I could sign the application form but not the reference. If there is nobody else, a good personal reference together with your dad's company reference should be ok. If in doubt, get the authorised signatory at your new employer to speak to their account manager at the airport ID centre.