Can i use 6mm cable for my 9.5 kw redring electrical shower?

Can i use 6mm cable for my 9.5 kw redring electrical shower?

The cable is already fitted but i want to upgrade to 9.5kw using a 45 amp type b circuit breaker . Most of the cable run is clipped direct ie ran up an inside cuboard and then into the loft before being enclosed for the last few meters . I am dis-agreeing with my dad because according to the regs clipped direct rate 6mm at 46 amp but enclosed is 36 amps who is right .(by regs i think the cable should be 10mm )

thanks for answer jim . I considered the cable run ,its less than 15meters .threfore the cable volt drop is small .