The runway capacity for each season is declared by Airport Co-ordination (ACL) after consultation with all stakeholders. The number of slots allocated at peak times takes into account an average delay figure, which I think may be 20 mins at EGLL. The term "average" could mean that sometimes delay is more than the agreed figure or sometimes less. The term "delay" I believe means in the case of departures the waiting time at runway holding point, in the case of arrivals holding in the stack.
Each day is different regarding delays because of many different circumstances, eg weather, go-arounds, switching runway direction, aircraft emergencies, ATC issues outside UK etc.
Allocated runway slot times are closely monitored by ACL and airlines which consistently abuse their runway slot times for no good reason could be the subject of sanctions imposed.
ACL website has lots of info on this.