"The cable size must also be considered, depending on the product and site conditions. It is now recognised as good practice to install 9.5kw and 10.5kw rated showers on a minimum cable size of 10 mm squared."
Amazing what a quick Google search will show up !
The 9.5kw electric shower at my last house was on a 6mm2 cable, but it looks like it might be better to play it safe these days and stick to the 8.5kw shower you mention. Either that or upgrade your cable which might be preferable as I can't imagine that the flow rate from an 8.5kw shower will be that great.