UK Data Protection law and GDPR
I had heard that the PNC has not been compliant with this legislation for years.
Scuttlebutt has it that it was the implementation of rules to become compliant which went wrong, and that this problem goes back to sometime in last year so some data will have rolled off their DR backups. If that was the case, then the rules to make sure that data that should be completely purged, even from the backups may have been incorrect.
Under UK and previously EU law, the police to not have catre blanche to keep information gathered from suspects forever. If someone is arrested for a crime, then the police can take and store information about the person. If they are subsequently released and the charges dropped, or go to trial and are cleared of the crime, then the police are bound by law to delete the fingerprints, DNA and other data that they've collected after a certain period of time.
Anybody convicted of a crime will have their records stored forever (incidentally, this is sometimes quoted as being why police will take fingerprints from people who are stopped for minor motor offenses, even when the data has no bearing on the crime - "it's standard procedure, sir"). Once someone has been convicted, the police have the right to keep any data they've collected whether it was needed or not.
This deletion policy does not please the police. They would really like to build up a complete database of all the people in the country whether they've been found guilty of a crime or not. If I were to put my conspiracy hat on (the one with the tin foil lining), this news story could be a deliberately created attempt to shock the people and government into a policy change to allow them to keep more information for longer.
Within the last decade, I heard a broadcast interview about the police's DNA retention policy with who ever was the chief of ACPO at the time, where he repeatedly called people who had been arrested but not yet tried as "criminals" instead of "suspects" (not even "potential criminals"), even after being pulled up by the interviewer on more than one occasion. I'm sure that some members of the police regard all of the public as criminals who just have not yet been caught yet!