I guess the OP is fairly new to this line of work and maybe only has had experience on new builds or recent installations and so have just had their posterior bitten buy exactly the sort of thing that BPG#1 tries to warn about. The OP's post does not make it clear if the cable has CPCs but they are not end-to-end connected, or if the cable is pre-1966 and has no CPC present.
Just for background information for the OP: the statements about ADS really come from the basic premise in the design of compliant circuits that no
single point of failure should cause a risk of electric shock. So with a class II product there are two insulation barriers, so damage to one is not enough to allow contact to a live conductive part. In class I products there is a CPC and so in the event of a L-metal fault then Zs should be sufficiently low (and cable adiabatic limit sufficiently high) that you get Automatic Disconnection of the Supply (ADS) by the OCPD or RCD so no person is not exposed to the risk of a shock for more than an acceptably short period (usually 0.4s here, TN & 230V).
Without a CPC then on the occurrence of a fault you get no disconnection. If you only had OCPD protection (probably the case for the old CU) then you never will disconnect and it is a
high risk situation. With working RCD protection you still don't get disconnection
on the occurrence of the fault, but should someone get a shock then they have a sporting change of not dying due to the RCD-based disconnection. The might just fall off a ladder of course...
So it is quite likely that you have made the system safer, but not actually as safe as the regulations demand. That is basically the just of the statements above that you can't certify it as it stands as "complaint".
Now I suspect the OP has just has an unpleasant lesson as to why most sparks on here always say to do an EICR first, or at least a few basic checks (e.g. as
@Pretty Mouth says in post #7). All of which is usefully stated in BPG#1 and that is also a good point of industry-guidance for any arguments, just as BPG#4 is for EICR coding, but in all cases it comes down to some judgment. But you are already in the position of the CU changed and no CPC, so folks pointing out the error or not checking first is not actually helpful. So what now?
The correct solution is to correct the absence of CPC connectivity. That could be fixing joints (if just stupid installation of T&E), a rewire if pre-1966 cable, or sometimes a separate CPC if a hidden joint/break cannot be otherwise fixed with an acceptable cost in electrician time and/or redecorating.
But the customer might refuse to pay for that extra work, or indeed they might refuse to allow work that causes decoration and/or building structural changes. Again, this is mentioned in BPG#1. If it is just a cost aspect and you think you can fix it without to much of a loss you might be best to just accept the loss was down to your own mistake and keep your reputation by fixing that side for free.
If however, they are not willing to accept the additional work 'for free' due to decoration/disruption/pigheadedness then you really can just make it as safe as possible and state in writing that you cannot complete it to the regulations due to their decision and advise on the use of class II accessories, etc, as described in BPG#1.
Just my 2p work, though I don't do this commercially so listen to others first.
