Some key insights from the article:
Basically, what they did was to look at how much batteries would be needed in a given area to provide constant power supply at least 97% of the time, and the calculate the costs of that solar+battery setup compared to coal and nuclear.
Less sun -> Less electricity produces -> Less electricity needs to be stored
Basically in Birmingham you need a lot more solar panels to have the same impact.
Ah ok that makes sense!