You mean 30mA - that will be about a required RCD, if a shower needed 30MA (Mega Amps) that would be one powerful shower!
The max capacity of 6mm cable depends on the installation method, and length. A quick calculation shows you'd only get away with 6mm for 9.5KW shower if it was clipped direct all the way, i.e. not buried in a wall / above a ceiling etc, which seems very unlikely, so you would need to either get a less powerful shower, or upgrade the cable to 10mm (and put an appropriate breaker in).
Note however that this work would most likely be notifiable under Part P, swapping the shower out using existing cabling etc may not be (I can't remember the exact rules), but certainly replacing the cable and breaker would, as it would effectively be a new circuit...