Walter, do the maths. If the 'clearance' was not above 500ft on the RPS, what would their actual altitude be based on the Aldergrove QNH? If they really did operate at/below 500ft on the RPS for the duration of the flight, there is absolutely no way they could have flown as straight line. It is probable that they did fly a straight line, but if they did this they did not stay at/below 500ft amsl. After all, once they were 9nm away from Aldergrove there was no 'clearance' for them to comply with.
Personally, I'm getting a bit bogged down in this whole Mitchell report thing. My original point was that the report contains so many assumptions (and errors) that any conclusions taken from it must be questionable.