You cannot enter the BHS Stage 2 exam directly (missing out the Stage 1) just because your instructor believes that you are capable unfortunately (would save us all a lot of money!!). You have to take the Stage 1 before the Stage 2 UNLESS you have successfully completed an NVQ level 2 or 3, or equivalent riding club/pony club exams.
I took both NVQ and the BHS exams and personally wouldn't have missed out any of the BHS exam days as they are an experience all on their own, and without going through all of them systematically I don't think I would have known what to expect or been fully prepared for each one (even though this meant spending more cash
The stage 1 day is love because it is so simple and uncomplicated, but will still give you a feel of how the days run.
If you are ready for the Stage 2 (and have the cash!!) maybe you could plan to take the Stage 1 and then the Stage 2 shortly afterwards??