Doing a top-of the head estimate in the spirit of Fermi's method ( https://en.wikipedia.org/wiki/Fermi_problem ):

There are 32 London boroughs, and several "primary business and commercial street" per borough, call it 3 or 4?

So about 90-120 high streets, as a ballpark.

The Article says: "You could argue that makes a total of 103 High Streets."

So close enough.