The problem is breadth not depth. In the case of a skyscraper that requires more power you then have only to upgrade a localised substation. In order for the entire country to consume considerably more _current_ that requires a global upgrade everywhere. There are 400,000 substations in the UK.

I'm also not saying it's impossible, it's just something that the govt has to do first for this to even be feasible. It also makes the timeframe very impractical. The govt doesn't appear to ever address this or even begin the work required.