Shower cable upgrade from 6mm to 10mm

Mainly work in scotland, dont do much Part P stuff, but your just changing the cable size so i cant see why you would need to notify. Its not a new circuit. Just my opinion, not fact.

To rewire a circuit here in England it is Part P notifiable. It’s also in a bathroom, and with the shower being located within Zone 1, it’s Part P notifiable. Be mindful of the cable installation method that you choose for the 10.0mm conductors. This will affect the current carrying capacity of them, and will place limitations on choosing the correct OCPD. Other circuit design parameters also require consideration. Regards, Carl.

Yes it's notifiable under part p as your are altering a circuit. It comes under a minor works certificate.

Any modification to a circuit in a special location as in a bathroom will need to be notified to the local buildings control office. You are changing the supply cable which in turn means routing, terminating testing and commissioning what is essentially a new circuit, even though it was there in the first place.