To be fair, we don't know for sure what is (and isn't) downstream of the CU in the shower circuit. In the 'transition period', it was not uncommon to fit an RCD in the vicinity of the bathroom when a shower circuit was added to a non-RCD-protected CU.A shower with no RCD protection ... not good. Okay, it may have been permitted 20 years ago, but you wouldn't want to leave it like that now.
The regs certainly would now require RCD protection for a new shower (or, indeed, for anything new in a bathroom - even if just a light or fan - and a new cable to a shower would probably also require RCD protection), and we nearly feel intuitively that it is not only a 'good idea' but 'highly desirable' (or 'essential'!) that a shower should be RCD-protected (on the basis that wet naked bodies and electricity are a bad mix!). However, I'm not aware of any real evidence that showers actually do present a significant hazard or that anyone has ever actually caused an RCD to operate by touching some conductive part (if they can find one) of a faulty shower. I suspect that there are more injuries, and probably even more deaths, due to people slipping in (or drowning in!) baths than because of anything to do with electric showers!
Kind Regards, John