Internally, it really varies from school to school, but I think A-range is normally about the top fifth of the maximum - so in an assessment marked out of 10 it'd be 9/10, in an assessment marked out of 15 it'd be 13/14/15, and so on.
Externally there is no A, B, C, D, E, just bands E1 to E4 (this being a one-unit subject). The top band is 45 out of 50 and above, I think.