This thread must have been responsible for a situation I've just come across.
There's a 7.2kW shower in the bathroom at the moment, and the owner wanted to know if he could upgrade to a higher rating as he remodels the bathroom.
Taking off the cover revealed 2.5 T&E feeding the unit. The isolation switch outside the bathroom was an older MK 20A type, but fitted to a modern plasterboard box. The feed to the switch was also 2.5 T&E, which when I traced back was spurred off the ring.
Could it have been the original immersion heater circuit? Maybe, except there was another switch adjacent feeding the existing water heater.
It looks like another typical bodged job all round. Apparently the shower was installed about 7 years ago by a "professional."