6mm cable has been used on 8.7kw for many years. Needs a 35amp circuit breaker but existing 32a will work OK. problem with instant showers is Winter cold incoming water. Summer water temperature will allow a lot more flow.
I built a shower room and took out my bath but still using a hot water cylinder so i was able to fit a double ended 2 Bar pump for a flow that will bash you up if used on full. I put this in for future as i knew a bath would be unusable in later life so i installed it while i could still do the work myself. Cost was about 1200 pounds with the glass screen and very solid bath sized base with just a 1 inch step. I ended up liking a shower now.