One thing I would say, on the basis that cables can only have too little capacity, not too much, install the fattest cable that you can get into the connectors in your CU & shower. You may think that you only need 6mm to feed an 8K5W shower, and (derating calculations aside) you do.
But showers are getting bigger all the time, and when you come to replace yours (by which time you'll have decided you want a hotter one) you'll be glad that your wiring can cope. You can already get showers which consume 10K5W (and, for all I know, more). Again, ignoring derating, 10mm cable is good for 53A, 12K75W, which is probably enough, but then Bill Gates once thought that 640KB was enough memory for a PC.....
At 70A, 16mm may be a step too far, and even if it fits, it'll be bloody difficult to pull, but if it's practicable, why not?