You're right, I've done a bit more research into this. The company in question (Ubitricity) are uprating lampposts to 25A single phase at 240V, so 6 kW per post. Even though sodium (and earlier flourescent) lighting never used anything like that much current, I can't find a source to confirm whether or not the lampposts were originally rated at 25A or lower.

On your calculations of how many cars can charge at once, I would keep it simpler and say that a row of parallel parking typical in UK cities, there is one street lamp for every 5 cars. 50 kWh charge overnight, approx 300 km range for every fifth car.

Supposing 100% EV adoption - well, already not everyone drives themselves to work/kids to school in UK cities (where most on-street parking takes place) - but if they do, and have charging at work plus some nearby rapid charging, that should do the trick, assuming a typical commute by road isn't more than 50 km each way. It's certainly in the ballpark.