April 12th is a key date for city centres (and suburbia, to an extent) with pubs and restaurants scheduled for reopening in outdoor areas, and non-essential retail likewise. Although the high street has lost a few names in recent years, non-essential retail and eating/drinking still constitutes most of a city centre's footfall outside of office hours.
Therefore, 29th of March for uplifts would be very sensible, as that's only a fortnight before, and actually only eight working days. We don't want to be in the predicament of rammed trains in April and May, when TOCs do not have the excuse that they didn't know about this back in late-February.
One other thing that needs to happen - last trains need to go back to pre-COVID times, given that there will be no 10pm/11pm rule anymore.