then do it with a dedicated 45 Amp Double Pole Switch in the
kitchen and use it as the maintenance isolator, that way you
won't need to fit any more switches into the circuit.

Only if it has holes to fit a padlock in the off position.

Also, make sure you fit a 30 Amp rewireable fuse or a 32 Amp
type B Miniature Circuit Breaker in the Main Consumer Unit.

These will allow between 7.2kW and 7.7kW, if we are generous and allow 240V
for our calculations. The shower is 8.4kW. I'd go with a 40A/30mA Type B
RCBO. The thought of having an instananeous shower on a rewirable fuse with
no RCD makes my spine shudder (and will probably make your's shudder pretty
uncontrollably in the event of an earth fault).

I haven't got the tables in front to check the cable's suitability. 6mm
seems marginal and certainly needs to be calculated properly to be sure it
is suitable.

Christian.