I would just like to second John Halucha's comment that the 1st April was just a nominated date with the Polish army came under British Command. My father's (Wladyslaw Pacewicz) MoD records state that he was transported to Persia on the 24th March 1942 (the first shipment date I believe) and that,
"Together with the Polish Army units, crossed the Soviet-Iranian frontier, was evacuated to Iran, thereby came under British Command with effect from 01.04.1942."
Quoted Margaret Goddard, Ministry of Defence, UK
It is a wonderful initiative - well done!
Kind regards, Anna