Can we go shorter? The Director General of MI6 traditionally has the designator 'C'. This probably wouldn't work anywhere in the UK, but if you dropped it off at a Post Office near Vauxhall Cross, they'd probably figure out what you meant.
Since not including a stamp should imply it's for an MP maybe it would be enough to choose a unique short name. I think there's more than one Ed, but Lia or Naz should be sufficient.