Sewell Road is the main National Grid transformer station for the Plumstead / Thamesmead area.
A 2006 document http://legacy.london.gov.uk/mayor/planning/srdf/docs/east-srdf-annexes.rtf reported that by 2016 it would need to be upgraded from 4×15 MVA to 3×60 MVA transformers, the incoming supply being increased from 33kV to 132kV, to meet the increased demand from housing developments in the area. In this photo there are clearly four, not three transformers so it is not clear whether this upgrade has taken place yet.
To the right is part of the Thames Water Pumping Station. I have not been able to discover anything online about this facility, although I guess that it pumps sewage from the local area (Abbey Wood) into the Southern Outfall Sewer (the photo being taken from the Ridgeway path on top of said sewer).