GT3
I'm not even sure that you are "technically" an incursion if you cross the Cat 1 bar before the preceeding traffic has been given take-off clearance. "Departing" doesn't seem to be defined anywhere, certainly not in CAP 413. It could be receiving clearance, starting to roll, applying power or entering the runway. I go with whichever suits me best on the day!
G W-H