hey - i'm familiar with the british o (equiv to our 10th grade) and a (equivelent to a hypothetical 13th grade) levels, and took the mcat. the mcat has 3 sections (physical, verbal, 2 short essays, biology) - the one major difference being that they are all multiple choice. they are also mostly analytically based from reading a passage and responding appropriately. each section is out of 15, and the max score is 45 (3 sections - for 2003) and a writing score. its a challenging test, and getting the highest marks when you write it can be more challenging than top marks in your a-levels. its also like comparing apples to oranges - the a-levels can be pretty difficult too depending on the subject. i imagine you are taking biology, chemistry, and physics as your 3 a-level subjects? make sure you do a lot of outside reading - the verbal section can be challenging for those that aren't use to it. goodluck.
-jot