Because this is the first year that Birmingham are using the UKCAT you can't be too sure about what a specific score for interview might be. The admissions site says there isn't a minimum cut-off, only that higher scores will give an advantage (as you'd expect).
It's probable that UKCAT scores will be used to choose between candidates in deciding who to interview, as pretty much everyone will have the same GCSE and A Level grades. They say that the SJT band is used at the interview stage, so it's likely that it will only be used if they are having an issue choosing between basically identical candidates.
My advice: don't worry too much about the UKCAT score right now, just be sensible about where you apply if you don't think your score is very good.