Instant shower wiring surface mounted?

[ABCwarrior]

Is it possible to install an instant shower and keep the wiring surface mounted- if so what is the legal/safe way to do this?
Our house has a downstairs shower working off the mains water- fine if the multifuel stove has been on as it is in the winter-free hot water, the solar panel in summer can't be relied on here in Scotland and rather than waste a fortune heating the immersion, and the lack of it being instant(awkward timer to programme and with us both working continuously changing shifts it's easier to boost it manually). But rather than run 3kwx2 for an hour, 6kw, a 5 minute shower from an instant would be 1/12 of whatever it was(9kw? so 750w of power used?)
The shower cubicle has the room to mount a shower on the opposite wall to the thermostatic one, but it's been nicely tiled, it's external so don't want pipes in the cavity anyway so they'll be tastefully surface mounted.
It's just the electrics that concern me as the entry point to the shower unit would need to be condensation proof surely? Or is this just not done?
It would maybe look a bit strange but it's the best of both worlds- when it's been sunny or the fire has been on, free hot water so use the thermostatic. Been out for a couple of days and overcast and/or winter- use the instant no waiting about for hours?

 

[securespark]

Not sure there's a way of tastefully mounting cables and pipes on the surface?

What size shower are you thinking of? What arrangement do you have in the way of consumer unit(s), incoming supply etc?

 

[ABCwarrior]

The shower would be on the same level as the CU, same height really. The CU is in the "utility" room which you go through to get to the downstairs toilet/wc/shower. It would be a 9kw shower, cable run of about 4-5m max, passing through one stud wall. Unfortunately when they installed the lower Immersion element as an afterthought they used up the last spare slot for an RCD, so it means installing another smaller CU(maybe with spare slot to take an external cable eventually to a workshop/garage for power). Water supply probably from above entering shower underside(bath is directly above can go through the floor?) to keep pipes to minimum.
We were told initially cos this is meant to be an eco house(! my a**e) that we did not have enough power from the transformer locally to run a shower. Baffles me that we could have both immersions on, cooker on full pelt, a kettle, plus a couple of 2kw oil radiators on the go though!!!
It wouldn't have a string/pattress box thing just a 45/50A cooker switch for isolating it outside in the utility room, probably next to the CU.