when insulating a loft any wires must sit on top of the insulation not underneath to prevent overheating. what should be done when boarding the loft using chipboard-type boards? can the wires lay beneath the board and on top of the insulation or should they be exposed?
In the 17th Edition Table 4A2 gives 4 installation methods 100 to 103 here it shows clearly how each way of running cables and Table 4D5 then shows for each method the current rating of the cable. Clipped to a beam will allow more current than in middle of insulation and clipped to other boards or ceiling more than center of beam etc. So for example 6mm cable it could be rated 23.5 to 35 amp. You may also find these tables in manufactures data like Batt Cables. Although clipping to top or bottom of the beam may allow more current it is also more likely to be damaged so keeping 50mm away from top or bottom will be safer for the cable. All a case of balance.
Sorry I suppose you wanted something more hard and fast but electrics not quite that easy. There are also calculations to do the same thing.