Let’s summarise here
All this is only considering the installation method, the regulations, my thoughts.
This does not consider volt drop or any other applicable derating factors.
Cable run 15m of 6mm[SUP]2[/SUP] T&E all installed to reference method B (in my opinion)
Shower 9.6kW @ 230V or 10.5kW @ 240V
Protection device not yet selected.
CCC for 6mm[SUP]2[/SUP] multicore 70[SUP]o[/SUP]C
Ref B 38A
Ref C 46A
Current demand of an instantaneous water heater (no diversity permitted)
9.6kW @ 230V I=P/V = 41.7A
10.5kW @ 240V I=P/V = 43.75A
So in order to carry a current of maximum 43.75A then a 6mm[SUP]2[/SUP] cable must be installed to reference method C and cannot be installed to reference method B.
Questions you have:
Can I use a 45A MCB?
For the shower as an appliance and if the cable were suitable – Yes
For 6mm[SUP]2[/SUP] T&E installed to Ref method B – No (also you cannot use a 40A MCB)
For 6mm[SUP]2[/SUP] T&E installed to Ref method C – Yes
What is the maximum length of trunking before derating is required?
The regulations do not state a minimum length before derating is applied.
You could possibly consider as an electrician that the regulations do give variable derating factors for different lengths of cable in insulation. If you were to apply the same considerations to trunking this would mean that full derating is applied if the length is greater than 50cm. As your length is 200cm full derating would apply.
Solutions available:
Use the 6mm[SUP]2[/SUP] cable as a pulling wire to draw 10mm[SUP]2[/SUP] cable in to replace it, use a 45A MCB, upgrade bonding.
Decide that under the floor is reference method C, remove the trunking, use the existing cable, use a 45A MCB, upgrade bonding.
Read the manufacturers instructions and decide if they specify that you can use any other method of safe installation as these would override the wiring regulations.