BS5839 talks about adequate support, so the question is, in a closed ceiling space with no access is fixings at 500mm intervals with every second fixing being fire rated adequate, personally I think possibly so.
BS7671 on site guide table 4A provides spacing, however this talks about cables in accessible positions, so if the cables are not accessible and unlikely to be disturbed then it is arguable that these minimum fixing spacings are not relevant?
I'm looking for some input please.