Are all showers of a 40amp breaker and out of your board do u just got straight out to your shower isolator close to shower because someone told me you have another isolator local to the board and is it best to wire in 6 mm unless it's over 8 kw or abouts?
Cable size depends on rating of shower and length of run of the cable, provided there is RCD protection on the board you would run straight to the isolator in the bathroom.
As above and 10mm would be better for customer that way they can upgrade shower without extra expense or danger to themselves if 6mm is not big enough for new shower.
Always use 10mm if you look at the de rating factors for cable covered in more than 100 mm loft insulation method 101 cable incontact with plaster board ceiling covered in thermal insulation exceeding 100mm 10 mm cable is rated at 36 amps so should be protected by 32 amp MCB , dont forget all circuits in the bathroom need RCD protection not just the shower infact for a 10.8 kw should be wired in 16 mm 10800/230 = 46 amps only cable on installation method 101 is 16 mm then a 50 amp switch to
info table 2.3.1 page 31 electricians guide to building regs
so dont know about your areas but they are putting 150 mm loft insulation all over the place here
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