[baakyocalder]

The Chantry system in Mage has always been wonky, even when the Book of Chantries was the rules set used.

I can see Chantry as having two functions not covered by other Backgrounds:

1. Access to other mages who are not mentors but who could provide assistance. These could be Allies or Backup or other backgrounds, but the power of a chantry is based on its membership. So, like being part of a squatter's chantry with a few weak mages is worth little but being part of an ancestral chantry like the one connected to Second World of the Dine, an old Dreamspeaker chantry, is worth a lot.

2. As noted by whwhs, you could also be buying real estate. Since there are merits for Property and Safehouse in Mage 20: Book of Secrets, which are similar to those in the Book of Shadows, real estate isn't a bad thing for chantry to buy. Sanctum, where the mage's paradigm works, might also be part of this background.

As for electrification in London, two books I read during research on the early electric industry could give you some background on electrification in early twentieth century Britain:

Thomas Park Hughes follows a systems approach in Networks of Power.

Alfred Chandler comparatively discusses the growth of electrical equipment firms and thus a little bit on electrification in the U.S., Britain and Germany in his tome Scale and Scope.

Oh, and a boardinghouse in a diverse neighborhood sounds like a great way for a chantry to form in London.