Choosing correct cable for shower in bathroom

[tonk]

8.5 kw
/ 230 = drawing 36.95 AMPS
Best MCB = 40amp
My cable will be leaving the CU, next 4 metres in conduit in a hallway, then through the ceiling to first floor. Under the floorboards for another 1 metre.
From this point another 2 to 3 metres to the above shower.
Method of insulation B is the best fit in my eyes
Appropriate cable size 10mm2 according to table 2.3.1 = REGs
Assuming that the temperature will not be great using B method.
Can you please confirm that the above is correct - Great if you can

Second question: shower isolating switch that I will install outside the shower, what rating should it be?
Thanks

[Someone-Else]

I presume you mean Trunking not conduit?

Trunking being white, with a removable lid on its length. (you can also get it in red and black)

10mm will be fine, you can also use a pull cord for isolation, I would opt for a 50A one

and RCD?

[tonk]

Trunking - you are correct and thanks
Thanks for other info, all makes sense.

Tell me, understanding I now know what trunking is: Rectangle shape with removable lid --- What is condit please.
thanks

[Someone-Else]

No idea what condit is, but conduit Click here