Power supply installation for storage water heater
Ampacity, IECland style

The IEC equivalent to the North American NEC for low voltage (&lt;1kV) installs is IEC 60364, and its ampacity tables are in part 5-52, Annex B.  Since we're dealing with multi-conductor cable, we make the conservative assumption that it's installed directly into an insulated wall i.e. condition A2 (any other condition, such as a cable in a surface raceway, yields better cooling of the cable i.e. higher ampacity in the IEC world).  Also, since the water heater in question is cord and plug connected (vs. hardwired), we assume no ambient correction from 30degC ambient is necessary.

Since this is a 15A circuit, copper wire is recommended -- aluminum is a bit fiddlier to terminate (even if it's a good alloy i.e. AA-8000 series) and offers no real cost savings for a wire this thin.  PVC-insulated cable is fine here as well, unless local regulations require rubber or XLPE (i.e. thermoset) insulation instead.

Given all this, we get a rating of 14A for a 1.5mm^2 cable.  If this was a hardwired heater, we'd be done.  However, the heater uses a BS 546 equivalent (type M) 15A plug, so a mating 15A socket is needed, and the ampacity of the wire feeding the socket must be greater than the socket.  Hence, we go up to 2.5mm^2, the next standard wire size, which gives us an ampacity of 18.5A.

So, get the 2.5mm^2 multi-wire cable.