I was thinking that if the wall had sufficient glare, it could possibly be that bright.
However, I found a bigger issue. I was re-watching Study in Pink and there's also a window on the staircase landing, one that's very clearly an exterior window. The way the flat is laid out, that leaves pretty much no room for a bedroom or bathroom. The flat is definitely on the second floor, with two floors above it, so there's no way that much light could penetrate 2 floors.

It's annoying, because were it not for this window issue, Ms. Hudson's flat below it would actually make sense layout-wise (the chimney, however, would not). The layout of 221C, however (the flat where they found the Powers' shoes) makes exactly no sense. They go downstairs, to the basement, and yet there are windows on both sides on the living room. One of the windows is high up and small, and makes sense for a window, but the other is full length... The location of the room relative to the rest of the building also doesn't make sense.