Given that the flowrate is calculated at 40C, why would you be adding
cold?

Do they say 40C temp rise? Well 16 litres hot only, which is a very good
shower indeed, and too much for most people.

No, I mean at a 40C output temperature. It is standard nowadays to calculate
using 35C delta T, equating to a incoming temperature of 5C. Summer flow
rates will be higher.

Christian.