The expansion of flight times to allow for delays spread outwards from LHR, I think. Now everyone does it. The other key change was placing the listed departure time much earlier, to try and ensure that Pax got to the gate on time. People started to stay: "We took off 40 minutes late ..." which they did not. You cannot detail all the times for Check-in; Hold; Gate; Doors; Off Blocks; Runway times.
The focus of this started when:
- Consumer groups and govts started ranking carriers in punctuality
- CEOs and Boards of Directors had hysteria
- Crews were told to hurry things along
- Crews pointed out that the whole mix of systems in Europe was a bit more complicated than that
- The kids in Marketing quickly thought up things
- Problem solved
- The media / govts / Directors moved on to the next panic.
Yes, I know that's a rather casual way of looking at it but I've worked in many corporate spheres for over 40 years.