CC Volunteers can set up the defibrillator but not press the button, basically. Each flight will therefore need just the one 'fully' qualified crew member to do the work, but to quote the AVMed instructor,
"You are being trained to the same standard as other cabin crew, but because this is a 1 day and not a week long course, the CAA felt unable to give it full acreditation"
The instructor said it was a stupid decision by the CAA, but they are the regulators and what they say, goes.