I have wondered about the same thing...I think I know the answer after talking to an ATCO. Yes, you are cleared to cross the holding-point line, but not until the departing aircraft has completed the take-off. But this is obviously not very convenient. And the reason to why the ATC gives such a clearance, is that it releaves them from the responsibility to wait until the departing aircraft has completed the take off.
You might think..."But I've got my clearance, why can't I cross that line?"
Think of it this way. You get the clearance: "Behind traffic on final, line-up and wait behind."
Same thing, you have the clearance, but you have to wait.
I must add: I am not a whizz and if I am totally wrong,
please correct me.
Cheers!
