OP
J&M Electrical
If the cable is clipped direct then as Table 4D1A states the cable is rated at 47 amps.
Chances are though that the cable is run from the Pull switch through the loft under the insulation therefore reducing the current carrying capacity drastically. The OP could clarify this.
We have always (old company policy and what we see as good practice) run any shower >9.0KW on a 10.0mm cable or chose a different shower. Better to be safe than sorry