I think there was a reasonable expectation by the owners of this data that it would be kept. The Home Office was obviously aware that it was still being used. The DPA requires that data is processed fairly and I would suggest that destroying this data does not amount to fair processing.
I believe this destruction was a contrary to the DPA. If this is the case the Civil Servants who made the decision to destroy these landing cards may have committed the criminal offence of "Misconduct In Public Office".