Running 2.5 ring main cable under a floor & over a ceiling

Please, what is the correct method of installing a 2.5mm2 ring main cable under a bedroom floor?

The floor void is 150mm deep with timber joists on 400mm centres. Underneath is a 12mm plasterboard ceiling. The void is mostly filled with fibre glass insulation, hence my concern about thermal insulation in close proximity to the cable.

Possibilities seem to be:

1. Lay it flat on the plasterboard

1. Pin it to the side of a joist with Tower clips - but if so how deeply down the joist? Does a 50mm clearance below the floor (which the clearance specified for cable passing through a joist to avoid nail penetration) have to be allowed?

2. Run it in mini-trunking or some other type of conduit?

1. Some other way? Going round the room above floor level is not a practical proposition.

In any case what would be considered the normal method if the fibre glass were not present?

TIA for help & advice.