Your parent wasn't talking about prime arable land, they were talking about city land.

The point is the electric delivery problem is much cheaper if the power generation and the power delivery both happen within the same substation (eg in the same neighborhood). If the generation happens in rural Scotland but the consumption happens in London, then all of the transmission needs to be upgraded. The point of your parent's comment was to describe a trick to minimize additional transmission costs/development.

And London area land does cost, even if it is just borrowing a vacant building's rooftop.

I don't necessarily agree with your parent's comment, but I don't agree with your comment either.

Except the worst case demand has to be catered for -- overcast horrendous rain in winter with local solar producing near nothing yet local demand (EV, Oven, Heating, Shower) all running.

You don't get a benefit of reducing peak demand from on-building solar, so you can't do with smaller substations.