I think this was a pragmatic decision based on the fact that the quicker you get to achieving a herd immunity even if imperfect might reduce the transmission to a greater extent than achieving maximum immunity in half the number thereby not achieving a critical level of herd immunity.
Also I thought nobody has yet had the Astra Zeneca vaccine and if some had on the basis of having two shots then I can’t see why this couldn’t be honoured in this small initial cohort.