[ppf]

As I understand it, the UK's original street lighting served the dual purpose of absorbing the night-time production of coal power plants, which are hard to throttle. Now that we barely use coal, and instead regularly throttle our gas plants, there isn't the "leftover" current that you think.

[cjrp]

I interpreted it as having the infrastructure to deliver ~7kW to the kerb, which is a hangover from the previous lighting systems.

[ppf]

As parts and systems (cabling, transformers, etc) get replaced, I have no idea if the new parts will maintain this "excess current" (assuming OP really was talking about that), but even if they do, the power still has to some from somewhere.