Is Collaborative Decision Making (CDM) in operation at LHR? If so the Border Force should be able to see potential problems on the day before they might happen and arrange their contingency planning in conjunction with all the other agencies such as the airlines and airport.
But it seems to me that their flexible response isn't that flexible if people have to drive up from Dover, park up, get to the work area and start. What's that? 2-3 hours to get into position at LHR? A situation can easily develop within that time period. Everything is on a knife edge and one delay can have a ripple effect that goes on for hours. And you've just had a stressful commute and then are under continuing stress when you are there.
And what happens then at Dover at peak periods with staff redeployed where I guess they are much less able to predict demand?
It seems to me that all the cuts that have taken place in recent years (in this area and others) have meant that the flexibilty to respond to unusual peak demand is almost non existent and one solution is to have more trained staff. But that costs more money, so I guess the travelling public will have to get used to longer processing times as it becomes the norm. You could always downgrade the declared capacity to maintain service standards but that will never happen; it's too complicated.
As this is at Heathrow of course, the issue has a high profile but these delays are happening at other airports too.