So long as the fasten seat belts signs are off, I don't think there's any specific rules about whether or not people can lie down on planes?
I mean even in employment terms... People would find it pretty WEIRD if they came into my work and found me lying on the kitchen floor on my break, but as far as I know, there's nothing in my terms of employment that says I can't so I can't be reprimanded for it if I decide to do it [emoji23].
The same would apply here. If it didn't say in BB's rules and descriptions of what the flight crew could do that this wasn't allowed, then they can't really be penalised for it.