There is no specific regulation that forbids an installer installing a cable within a cavity wall. However BS 7671:2008+A1:2011 and The Building Regulations:2000+A:2010 highlight a number of reasons why this practice is undesirable:
The cable may be damaged during installation - BS 7671:2008+A1:2011, 522.8.1 refers.
The cable lacks support and also damage may occur due to the cable's own weight - BS 7671:2008+A1:2011, 522.8.4 refers.
Thermoplastic insulated and sheathed cables (PVC/PVC) in contact with polystyrene may cause the plasticizer within the insulation to migrate, leading to softening of the cable sheath - BS 7671:2008+A1:2011, 522.5.3 refers.
Cables installed within thermal insulation will have their current carrying capacity significantly reduced, which may require cable sizes to be increased - BS 7671:2008+A1:2011, 523.9 refers.
The cable may be attacked by flora and fauna - BS 7671:2008+A1:2011, 522.9 refers.
The cable may be in contact with the outer and inner wall thereby providing a route for water - Approved Document C Building Regulations refers.
From above, it is easy to see that cables should not normally be installed in external cavity walls, but where it has to be done the installation designer may well judge that Mineral-Insulated Copper Sheathed (MICS) cable is a suitable choice in such a location providing its selection and erection complies with BS 7671:2008+A1:2011.