Pace, it's not a matter of accuracy of whether one is taking off or not at the point of making the transmission, it's a matter of conformance to CAP413.
Long time since I did my PPL, but at that time they didn't encourage the application of philosophy or logic to RT. You followed CAP413. End of story. There was a thread recently on here or Flyer where an ATCO was getting very shirty about DIY RT, and quoted this in particular.
As it happens, I've always said "rolling", but I don't think I should!