Both anchors are out and she looks like she either has a stern cable out aft or is fast to a buoy, so presumably she's moored midstream. No courtesy flag to be seen, just what looks to be Tango Echo (please pass at slow speed) on the foremast.
Right enough, not sure how I managed to mistake that!
Saying that, it looks as if that's cable coming from the fairleads than wire/mooring lines, so I wonder if the've broken the cable onboard.
Stan Mayes will know!
This is the Alert at Enderby's Wharf on the Thames at Greenwich. I came across something on the web that was from Jan 2011 that shows the towers forward of the bows that were used to support the spans carrying rollers for loading cable. On another site, Britain From Above, again it gives the location as Enderby's Wharf and the chimney was there in 1946. This site has some excellent arial photographs of the docks in London both pre and post WW2.