I never saw this fabled "drop" in insurance. When I was <21 people told me it was 21, when I was 21 people said it was 25. Now I'm 29 and I've only ever seen steady drops until now when I've plateaued.
I'll see what 30 brings, but by the looks of it, it will be an increase.
I've never been quoted more than £1200 to insure any vehicle I've owned though, even going back to 18 with a Saxo VTR.