I don't know the global rule (or a reference to it), but locally at LHR it is part of their "minumum runway occupancy time" drive that a conditional line-up clearance after 1, or very often several, departures does authorise you to pass the holding point line and get snuck up into the "neck" - actually you are requested to so, so that you are lined-up and ready to roll immediately you get takeoff clearance.
And as for the "single movement" comment, it is a periodic cause of confusion at LHR where you might have a conditional line-up clearance 5th or 6th in sequence from N and S of the centreline.