A lot of grey areas with a lot of yes and no answers I am afraid. If the runway is used for departures only and you are 100% sure that you are next to go (the single line up procedure at the moment should help here) then moving up to ensure best runway occupancy could be deemed ok. However technincally if you cross the CAT1 prior to the a/c being cleared for take off then starting to roll it could be seen as a runway incursion. You and the skipper are both right and wrong if you get what I mean.
Ever come up to the tower at LHR? More than welcome to visit us and see how we work in deps - might be a huge benefit to all crews.