Hi mate, it depends on how the cable is run.
i.e is it in conduit in the wall, does the wall have insulation in it? These factor can contribute to what size cable you need.
If your shower has a max power of 9500Watts it will be pulling 41Amps.
According to BS7671 the wiring regs, the current carring capacity of 6mm cable in a stud wall were the cable IS NOT touching the inner wall surface is only 23.5AMPS, and were IT IS touching is still only 35AMPS.
So it looks as thou your cable is under sized It may need changing for a bigger size i.e 10mm. Which could well be a pain depending on how far the run is.
The switch is the correct rating , but your MCB will need down sizing to protect the cable.
This is why you may not have noticed any problems, even thou your cable is undersized and your shower is pulling too much power it still wasnt enough to trip the MCB.
Edit: just seen Beavis' reply