when i spoke to the admissions guy at the end of the talk at this year's open day, he said UKCAT scores are put into quartiles, and points are awarded accordingly. and he said the greatest percentage of offers went to those with UKCAT scores in the top quartile, and the smallest percentage to those in the bottom quartile. but he emphasised that some in the bottom quartile did still get offers.
obviously, we don't know where the boundary lines for each quartile are. but all im trying to say is, since 600 is the average score, if the rest of your app. is strong, don't let this put you off.
he also said a good UKCAT score can boost someone up into being in range of an offer, but if the rest of their application is strong, the impact of this will be less, if that makes sense. ooh, and also, near the end of the selection process, im sure they mentioned they rank the borderline applicants by UKCAT score, and use this to decide.
sorry about the essay, this may be no help whatsoever but it's just some things that might be useful to take into account. but if you loved edinburgh, and know the rest of your application is good (as your GCSE results seem to be) then there's no reason why you shouldn't go for it! good luck whatever you do.