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You're right about the linearity being flawed, but I think there's an important and rescuable construct there. Humans do have ranges of illumination where we can easily accomplish certain classes of tasks - very few want to tat lace or operate heavy machinery by starlight, for example.

Also dismissing the 24 hour availability of labor

We aren't talking about installing a single light that we pump all of the world's energy into, making it ever brighter as we drive the cost/benefit further and further into the realm of diminishing returns. We are talking about scaling a few common "good enough for X activity" lighting setups across billions of installations, and that process is very linear.

So £40,000 per million-lumen hours = £0.40 per lux, no?

Today that metric might look 10x better, around 4 cents per lux

LEDs can put out 200 lumens per watt, 5000 watts of that for one hour is a million-lumen hours.

Assuming perfect efficiency, that would use 5 kWh of electricity, which is $0.40 total if you pay $0.08/kWh.

I believe that works out to .00000004 cents per lux.