I'm doing A Level maths in June this year (I'm in year 12 too), and I too am a little concerned about the grade boundaries. But Ofqual have said that they will supposedly allow a greater proportion of A*s this year, although this isn't confirmed, I've only seen mentions of it. Bearing in mind that everyone doing it this year is in the same boat, and everyone has access to only the two spec papers. When I apply for natsci at Cambridge, and if worst comes to worst I only have a grade A, hopefully they will take into account of the high grade boundaries and circumstances this year. But don't know about for maths.
What sort of scores are people in your class getting on mocks?