I don’t have any access to how UPC actually works, but I can make a guess!
The new boxes don’t actually record anything (unlike the Horizon boxes, which had their own HDD).
The next bit is me guessing, so could be totally wrong:
All they do is set a flag against the programme in UPC’s systems that you have wanted it recorded. So, popular programmes no doubt have a huge list of UPC account numbers against them. I would then assume UPC deletes any programmes after 7 days old that are not flagged as being “recorded”. The 7 days are to allow replay. Further, after 60 days any programme not marked by someone as “keep for more than 60 days” is deleted.
If all my assumptions are true, I can see no way that manual recordings will ever be implemented.