As we have discussed in a previous thread on this subject, my local Police Force did licence my dischargers as section one weapons.
This may have been due to ignorance rather than the correct interpretation of the law. This was also about 10 years ago.
As you say De-act Certs are not a legal requirement, but they are the legally accepted method of proving the appropriate work has been carried out. If you do not have a certificate and the Police disagree, you will have to argue your case in court.