Prysmian 6943X Black 3-Core 4mm² Armoured Cable 25m Coil (67843)

The question is somewhat ambiguous, as it is not clear which cable you are looking to replace with the armoured cable. However, regardless, if you are referring to the ring main cable that feeds you sockets (twin and earth), then whilst armoured cable does provide some shielding, it is not a suitable cable for this application If you are looking to replace the actual lead that connects the cd player to the socket, then that cable is a flexible type cable, and again an armoured cable is not a suitable alternative for the application. If it this the cd player lead you are looking to replace, then we we suggest consulting with manufacturer for a suitable alternative cable lead, specially designed for this purpose, or perhaps a specialist Hi-Fi magazine

Any of the 20S type glands such as code 25996 will fit this cable as it's only 1mm thicker than the 2.5mm equivalent SWA.

Depends on how much power you want to run. In general 4mm can carry up to 32A, but it really depends on how it's installed and how long the run is, certainly no longer than 35 meters. If your heater has a 13A plug then you could go with 2.5mm which can easily cope with 13A and be fed from a 16 or 20A breaker, again depending on how it's installed.

The approximate diameter of this cable is 15mm. Please choose clips accordingly.

This product is a 'coil' of cable in shrink wrap. It isn't on a drum. No problem to fit into a car boot.