Marleybone is the best spot I've found so far. I usually head into the Royal Museum and then go into the East and West Clocktowers. There isn't always Scrap Iron there, but when I need it, I simply enter, look around on the entry platform, pick up what's there, then exit and go to the next Clocktower. By farming a few times in each Clocktower, you are bound to get some and it doesn't take long.
A few times I've found it on either side of the Royal Museum doorway, on the outside in Regent's Square, when there aren't a lot of people there.