The best strategy for dealing with questions you can't answer, or questions that you think will take you a long time to solve, is to guess, flag and move on. This means picking a letter and answering all the questions you don't know with this letter, then clicking the flag icon on the page and moving on to the next question. It is a quick way to get through all the questions and you can go back to the flagged questions when you finish with your extra time. This also means that you won't leave any answers blank, also by guessing the same letter each time there is a higher probability of you guessing the correct answer. The UKCAT is a test of how quickly you can answer questions, so it is best to put an answer down for everything, then go back and work through the ones you found difficult.