well I can only comment on how it was in 1998 when I took it, but that wasnt the case at all. Everyone sat the mock at my school then all those who got an E or above could take the proper exam.
Only the people who were doing ok at A Levels (getting grade C or above average) performed well enough to take the proper exam. Out of the GNVQ people none even those getting distinctionsin their GNVQ performed well enough at general studies to take the proper A Level exam and it ended up that only about 12 of us took it so it certainly wasnt an easy option. Neither did it boost the points total as only a few universities accepted it then.