Bearing in mind the duty cycle of an electric shower, wouldn't
the overloading be tolerable, in practice? I know of at least
one installation with an 8.5 kW shower on a 32A breaker.

I'd be amazed if it didn't work, I have to say. I don't have the curves in
front of me, but suspect a 10% overload would take tens of minutes to blow,
if ever. It would still be very bad practice, though.

Christian.