Most airlines these days have short turnarounds and very little fat in the schedule. Whilst this may be a contributing factor no padding or fat in the schedule is going to make a four hour delay go unnoticed. There preceding two flights had turnarounds of 40 and 35 minutes which is in excess of the 25 minute aim. I would be interesting in hearing if the way an airline schedules flights is acknowledged as a contributing factor to a complaint. If it is then I see hard times ahead for the airlines and significantly increased fares for all.
On that note, surely if snow or LVPs hit LHR and there are severe delays then based on the notion above then although it is a weather delay I could still make an EU claim as British Airways choose to operate from such a slot restricted airport which is more prone to severe disruption than most?