At 915 Fort Street stands the American Savings Bank which doubles as the headquarters of Widmore Industries. A telltale pattern of inlaid circular lights gives the location away. This is the building that an angry and confused Desmond departs after his meeting with Charles Widmore in Flashes, Charlie busks here in Greatest Hits, and this is the location at which Charles Widmore receives Ben's call informing him that daughter Penny is about to be Linusized.
The Widmore Industries signs look to have been practical rather than CG. One of them is visible in a behind-the-scenes shot below.
Below photo courtesy @kiwilostie
... Charlie Pace, pre- Drive Shaft. These scenes were filmed at the intersection with Merchant Street.
The scene with Desmond and Eloise together on a bench was also filmed near the intersection of Fort and King. The Londony buildings in the background are CG.