Conformance hides the delay. With a 50 minute conformance, everyone on the plane now has to turn up 20-30 minutes before they would have done (to keep the same safety arround delays. If you plan to have 90 minutes of safety to get through security because of traffic, you now need to plan arrive 90 minutes before T-50 rather than 90 minutes before T-30).

If LHR was increased to 50 minutes, people would need to leave 15 minutes earlier than they would, effectively it's as if the plane takes off 15 minutes late.

This of course means LTN (and LHR) earns more money from selling coffees and duty free while people are stuck airside.