[robga]

The PDF of the book provides reasoning.

https://access.bl.uk/item/pdf/lsidyv3c0c937d

"Taking the City of London for the central division, round it we have grouped six similar divisions ; these, in their turn, being encircled by a group of twelve hexagonals. We have chosen St. Paul's Cathedral as the core, because there we have not only the highest ground and the most familiar object, but the acknowledged central postal, telegraphic, and telephonic district of the universal system. Should it be objected that Charing Cross— or even Victoria, the centre of Greater London— would suit as well, it is scarcely necessary to point out that until the conditions of the two localities are reversed, and Commercial London loses its importance, the suggestion must fail."

The book itself is worth a read. Each hexagon could be divided in six, it says (of course). Then each 6th into 16 triangles with sides a "48th of a superficial mile ... about twice the length of Waterloo Bridge and contain, say, 200 lamps each". He goes on to say each lamp could be adorned with these mapping notes as an "ocular demonstration" for travellers. An 1885 Google Maps.