Having a certificate proves nothing to an insurance company... they just require a certificate.
A similar analogy is that of having an MOT certificate for your car. Even though it's a test of roadworthiness... the minute you drive out of the garage, you could be prosecuted for having an unroadworthy vehicle ! The law requires that your vehicle is roadworthy and that it has a current MOT certificate. They are two separate things. Having just had an MOT may help you prove that the vehicle is roadworthy, but in itself, it proves nothing !
It is for this reason, that the only people I use to do work for me, are people that I trust to do the job... regardless of what certificate they might want to issue me.
I recall many years ago having some gas work done... and I knew more about the gas regs than the fully Corgi registered plumber !