At Turkey Street Station a brook flows pretty much beneath the station.
Just west of the station is an extra bridge over the river, which seems
to hold nothing except a small shop. I can't understand why a bridge
over a brook would have been built, just so a shop can be put on it. The
only thing I can think of is that the shop used to be a mill, but it
doesn't look like a mill.

Which makes me wonder, why are mills usually built astride the river?
Obviously the waterwheels need to be in the river, but the bulk of the
building could be on the shore, couldn't it? Putting the building over
the river seems an unnecessary expense... after all, most rivers do not
have buildings on them, except in Central London.