btw, academic scores make out of only 20% of the total selection scores from the admissions tutor.
the other 80% is your work experience, volunteer work, reflection on your work, extra-curricular, etc.
out of that 20%, about 90% is your A-levels or IB or scottish higher scores (% depends on which course you take as they follow UCAS tariff points). then the ohter 10% is your IGCSE.
as for the UKCAT scores, since this is the 1st yr of implementation, most unis are using it as a differentiation between 2 candidates with same selection scores. so obviously the candidate who scores higher will be prefered. However, this only applies to certain unis.
i know KCL, Queens, and few others, weigh the UKCAT scores very highly. so if your get only an average of 600, it will be hard for you to enter (i'm speaking for international students, i'm not sure about candidate with EU status).