Gonzo,
The potential 'problem' with a 'Land after' is that until you hit the deck, we cannot be certain that you will land
Thats an interesting comment, in reality you can't be sure that any aircraft is going to land until it does so regardless of what sort of landing clearance has been given. In fact even after touchdown, until the reversers have been deployed you can go-around, not likely I admit but you should not assume an aircraft will actually land just because a clearance has been issued.
From my point of view, when you are close to the guy in front a land after is a far better way of dealing with the situation and I would like to see it used more at LHR. At bmi we have a standing order regarding late landing clearance which reads:
A G/A will be flown if landing clearance has not been received by 200 feet AGL unless,
a) the runway is clearly visible
b) the touchdown zone is clear of aircraft and vehicles.
In all cases a G/A is to be flown if landing clearance has not been received by 100 feet AGL.
I think we all imply from that with a land after clearance, 100ft is the min. altitude that you can go down to without having decided to land.
As far as rolling down the runway goes, I never do it unless we have been told we can. I also don't bother asking, in my experience it almost always offered when available, same for block 79's off 09R.