In 2 HFT shoots recently that I've been in this came up. One I was marshalling at and a shooter supervising a junior and sharing a rifle ran out of air and asked to pop back for a re-fill. I saw no harm so let him but when he came back he was shooting out of order and another shooter queried it and asked if and why I'd allowed it.
Another, this was read out as part of the course rules with the proviso that to shoot out of order all the shooters ahead had to give consent.
I can't see any UKAHFT rule about this anywhere but what is the accepted practice amongst the big boys down South?