I was thinking of putting some of this up in a ceiling whilst I have the joists bare, from below that is, before putting new plasterboard up. Will it be ok to just lie any cables on top of it?
you can have cables on top of it (so any heat can escape upwards) or underneath (so it can escape through the ceiling), but not surrounded/buried in insulation.
if they're lighting cables, then there's a high chance they'll be ok regardless of any insulation. If there is a shower cable in there, it will almost certainly have to be touching/clipped to timber or plaster and able to dissipate the heat.