"'IF' the official did everything correct and they're encrypted then there's no problem is there?"
Lets, think..... YES. FFS YYYEEESSSSSSS THERE IS SOMETHING FUCKING WRONG!
If the individual has followed an approved protocol, they may be in the clear. But for the MoJ, pointing to the post office and saying "they lost it" is no excuse. They had the fucking data, they've lost it. Doesn't matter if they gave it to a tramp, to a motorbike courier, a postman, or a passing tinker, the wankers of the MoJ have pissed confidential data into the ether. I wouldn't trust donkey post for confidential material, why should the clowns at MoJ?
What's wrong with encryption and electronic transmission? The whole idea of taking digital data, burning to optical, and sticking in a post box is imbecilic for any organisational data, but there's a further implication about data security here, and that is that flunkies at the MoJ have the facility to export large volumes of data (which shouldn't be possible) and evidently without any effective audit and control (which is unforgivable).
Head should roll, and they shouyld be at the very least the head of IT for the MoJ, the head of IT security at MoJ, and whoever authorised the sending of this data by post. Of course, that won;t happen, and they'll continue to accrue an over-generous pension despite their incompetence.