One of the great things about this forum is that we can have a thread titled 'Southminster Branch Electrification A Waste of Money?', and another thread arguing for the electrification of all manner of branch lines. Withou a difference of opinion where would we be?
The Southminster line (annual usage 1.25m passengers) was electrified very much on the cheap - c17 miles single track and no feeder stations, it's basically 51 miles of wire and about 400 masts. It was an add on to the wider electrification of Anglia Routes in the mid 80s, with great economies of scale. As has been said earlier, combined with Romford - Upminster and Stratford - N Woolwich electrification it removed a small pocket of diesel working, taking out the need for a whole diesel maintenance facility for about 4 units.
Compare to what is seen by many as a 'must do' : the Uckfield line (annual usage 1.38m passengers). 25 miles of double and single track which would need new grid connections and about 8 substations. Cost: a lot. In terms of benefit for cost, there are far better electrification schemes to be had.
Perhaps BR got it right?