From the regs:
"The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another."
"In a building with a storey 18m or more above ground level any insulation product, filler material (not including gaskets, sealants and similar) etc. used in the external wall construction should be of limited combustibility"
"The external envelope of a building should not provide a medium for fire spread if it is likely to be a risk to health or safety. The use of combustible materials in the cladding system and extensive cavities may present such a risk in tall buildings."
Appendix A gives requirements for the definitions of resisting combustion, it's the amount of time a material takes to succumb to the fire. "Flammable" is not an acceptable level of fire resistance ^^