The City of London is a weird ancient administrative division. It's not the whole of the city centre by any means, and not at all comparable to San Francisco.
I'm not sure you're aware of what's being discussed.
The Square Mile is an area of offices, mostly banks and company headquarters. It's the area of the ancient City of London, the bit the Romans walled off. There are now very few residential buildings. There aren't even any national government buildings, the area was controlled by merchants and guildsmen. Most people commute to work there, but you can say something similar about any 1mi² area in the most central bits of central London — few people live and work in Soho, for example.
Canada Square is in London's second financial district — Canary Wharf.
Most people working in London do live in London. The population of London (meaning the people who pay tax towards the public transport, vote for the mayor etc) is over 8 million.
Many startup companies are not far from the City of London / Square Mile — 5-10 minutes walk.
Canada Square is a long way away from the City of London (aka "Square Mile"). The City of London (that is, the small area to which the particular election protocol applies) has a tiny resident population; its resident:commuter ratio is not remotely comparable to that of London or SF.
