I called Exeter and asked them how they use the UKCAT. They said they have a threshold that you have to get. 3A*s does put you at tier 1, which means that they won't rank you according to your UKCAT, however ypu have to meet the threshold. They do rank tier 2 and 3 according to the UKCAT. They use the personal statement when judging between 2 similar candidates. I spoke to them a while ago, so don't quote me!