I haven't a clue about the bureaucratic mumbo jumbo.
But the risk with a donation is
1. Bruising or other damage to nerves in the arm - obvious to anyone
2. Fainting - limited to no more than an hour after donation
Otherwise removing a unit of blood is physiologically beneficial - our haemaglobin is set too high and reducing it by a couple of units improves blood flow and decreases risk of clots. In practice all this is academic, but it can be argued you are fitter after donation once you have had a cup of tea to replace the volume
I would be interested to be told any logic to waiting 48 hours or indeed waiting at all.