You may be overthinking this as it seems the question is asking for minimum cable size and as clipped direct it will be T&E, therefore having determined that a 6mm T&E is adequate the 2.5mm integral CPC will be adequate also as with the low Ze and type of OCPD deployed automatic disconnection under fault conditions will be achieved well within 'spec'.
Also I think every electrician would just use 10mm2 cable anyway. That way there is room for a future upgrade to maybe a 9kw device