I remember years back (2005?) I was at the BALPA employment conference at Heathrow and the Virgin chappy made a brief presentation.
He said Virgin wanted Airbus / Boeing time because due to the lack of landing practice available on the line at Virgin, they needed candidates fully conversant with landing a jet before they joined Virgin. I remember it so well because he also said "Get all your flying out of your system before you join us, because you'll do next to none once here". Ouch!
His words, not mine, and probably way out of date.