Prehistoric Lavatory to Modern Bathroom : Questions re. Fitting Electric Shower?

Hi all at
My bathroom is currently just unplasterboarded 3x2s and a forlorn looking lavatory as I am in the process of fitting all the associated bits behind the walls and moving a doorway so as to accomodate a shower cubicle in a v small, previously non-standard, 19th century Lancashire bathroom.
The old (prehistoric should be a better word) watercloset/washing facility had gas lighting and copies of the racing post from 1951 under the bath...make of that what you will!
Anyhow, the ability to wash for Christmas dinner beckons so...
Since I cannot seem to find this info anywhere, could someone please tell me :
A) Does the cold water supply pipe for a 9.5kW electric shower HAVE to be copper or can it be 'easy to work with' plastic pipe instead? Saves me having to earthbond a copper pipe, seems easier to seal too and less like to expand/contract like copper in its hole!
B) At some stage a 10mm electic cable comes out of the pasterboard/tile surface behind the electric shower. Is there anything I should know about how this wire appears 'out of the wall'? Is the end of a 10mm cable ok just coming out of the wall? It needs to have sealant around it (obviously something or water will get into the wall space won't it?) but is there some special fitting/grommet/gland that 'real' fitters use?
I already know that the cable should come in from ABOVE BEHIND the wall, much like outside lights need a loop of wire dipping down behind them for water to drip off and so make it less likely for water to follow the cable run and go into the wall.
Not wanting water in my wall spaces, I want to do the right thing?
OR is the last bit of wire to the shower supposed to be outdoor waterproof cable?
Not fitted anything electrical specifically in a bathroom before, but am confident I can and do so safely, so any advice is greatly appreciated!
(Dirty Dusty) David.

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