http://www.cable-ratings.co.uk/
A 9.5 KW shower is nominally 41.3 Amps (at the UK standard of 230v).
However, the UK the voltage can (is allowed to) drop to as much as 216.2 v, which would be 44A.
A 10mm cable is on the limits if its buried in the walls, ok of its clipped externally.
Distance can be a factor, but only if you live in a mansion.