I didn't take grade 6. I took grade 5 and then 7, and am going to take grade 8 soon.
By grade 7, you are expected to be able to play up to the 9th position, meaning your first finger will be on the high G note on the E-string, in the 3 octave D major scale. By grade 6, up to the 7th position in the scales. Grade 8, in terms of reaching high position in scales it is the same for grade 7, but grade 8 scales are a lot more difficult because you are expected to play 3 octaves in one bow.
As for the pieces, i find grade 7 pieces manageable despite jumping from grade 5. As long as you know your fingerings up to the 9th position, can do vibrato well, have fairly good dexterity, grade 7 shoudn't be a problem.
I felt that the jump from grade 7 to 8 is slightly more than that from 5 to 7. By grade 8, the pieces requires a very wide range of articulation, such as spiccato bowing, marcato, sautille, a bit of left hand pizzicato, fast detache bowing, etc, etc. On the left hand, be expected to play a lot of double and triple stops.
Hope that helps.