To the best of my knowledge there is only one embassy currently in Lowndes Square, the High Commission for Pakistan. (A Google search for 'embassy "Lowndes Square"' finds nothing else, and this site has no other listing:
http://wwp.londonw1.com/embassy/index.htm ).
Some embassies raise their national flag each morning and lower it in the evening. If the High Commission for Pakistan follows this practice, and the Google Street View picture was taken very early in the morning, it might explain the absence of their flag.
Chris