If the supply cable goes through the loft at all i personaly wouldn't fit any shower above 8.5kw on a 6mm at all (even then it could be very tight depending on cable length.
My main reason for this stance these days is because i've been in that many lofts in the last few months which have had extra insulation installed (due to government schemes for free insulation) that gets thrown over all cable's and joists (so you cant see the joist just have to feel for them) that a cable close to the limit will soon be unsuitable through external influences. Not our problem some will say but if it all melts/catches fire who the first person they're gonna blame?....muggin 'ere and i could do without that hassle tbh.