Example - take a 3 kW immersion heater fed via 20 m of 2.5 mm^2 cable.

I'd say that is unusually long. Have you been taking lessons from dribble?

I doubt it is excessively long in many installations. Remember that it often
has to take a circuitous route, and perhaps go up a storey or two. Indeed, a
quick estimate on the length of my immersion heater cable is 18m.

Christian.