The UKCAT is not a test to measure how quickly you can finish it.
Each question counts, just as each mile in a marathon counts. Two hours may not seem much, but it will exhaust you if you go full speed from Verbal Reasoning, but run out of energy during Decision Analysis. In most medical schools, all subtests count equally (i.e. the use of an average UKCAT score), however; some may take your lowest obtained score for a subtest to vet you through for interview.
The most important thing to realise is that you don’t get points for simply answering questions. Missing out a few and getting the ones you attempt right would put you far above anyone who whizzes through without paying attention the exacting requirements of each question.