You are referencing "Need Prysmian" & "Insulation"... you mean using
6242BH?
6242YH is BS6004 70oC PVC insulated.
6242BH is BS7211 90oC XLPE insulated. The "H" means Harmonised colours re Brown/Blue replacing Red/Black for FTE.
If the cable is run against insulation such that one SIDE contacts plasterboard then normal cable is fine, because it can lose heat to the non-insulated side. If however the cable is run THROUGH insulation then 6242BH is used which has a higher temperature limit and thus maintains more Currency Carrying Capacity (CCC) after derating for insulation & temperature.
6242YH v 6242BH for example...
- 1.5mm - 20A v 24A clipped direct
- 2.5mm - 27A v 33A
- 4.0mm - 37A v 45A
- 6.0mm - 47A v 58A
- 10.0mm - 64A v 80A
- 16.0mm - 85A v 107A
Screwfix sells 6242BH in 1.5mm, 2.5mm & 6.0mm. It may be Prysmian, but you can use anyone's BS7211 and to be honest you may prefer to do so because Prysmain's 6242BH has an extremely stiff cable sheath & extremely stiff conductor insulation - it is very hard to work with compared to other generic brands I have handled.
For light final circuit cables through current insulation it is typically advised to use 1.5mm 6242BH - that has sufficient CCC for all possible scenarios. For a shower you may need to use 16.0mm 6242BH which is unwieldly and may not even fit the wiring accessories.
Note you can NOT assume a wiring accessory can handle 90oC, that is to say you can not fuse 2.5mm 6242BH at 32A - it must be fused to 20A (or
25A if clipped direct and you can find a 25A breaker/fuse). The purpose of the 90oC limit is to permit a "higher starting point" for grouping, insulation & temperature derating calculations so you stay above (say) 16A per leg of a ring final circuit.
Now, if you are NOT running cable THROUGH a body of insulation you do not have to derate so severely. Indeed some people run the cable on top of the insulation so it is at least visible and derate for the cable penetrating down thro the insulation depth. I think current regs for loft insulation are 380mm or such like, so the calculation would be 400mm.
You need a copy of the IEE OnSiteGuide - that will answer your questions re calculations (cable thro insulation). Amazon UK sell it cheaply, well worth buying a copy.