I can't speak for the departures, being an approach man (although I don't believe they do unless it is specifically asked for).
On approach we still treat the B757 as an upper medium (specific to the UK) meaning that B737s and similar get 4 miles. There are an increasing number of airlines however which now insist on Heavy spacing (5 miles).
Thus far the argument has been that no proven case has been made to change; the reality is that commercial pressure is now backed up against inadequate runway capacity.
Point 4
