Aptitude tests like the UKCAT and the BMAT are ostensibly designed to reflect innate ability, rather than preparation. There is also an element of luck: how sharp you are on the day, and the questions you draw from the question pool. But in practice, the margins between meeting and missing medical school cutoffs are so fine that preparation is essential for these tests, even if it just means becoming familiar with the format and developing a sense of the pace at which you will need to work through the questions.
However, in addition to this I believe that some degree of focussed practice is also beneficial in order to develop your ability in each of the individual sections of the UKCAT:
The verbal reasoning section is perhaps the most difficult ‘core’ section to prepare for, as it depends on literacy skills developed over years. However, the speed at which you must scrutinise passages for key information is so fast that it is useful to practice exam-style questions and then examine how you came to correct and incorrect conclusions - you are likely to find that certain errors of reasoning tend to come up repeatedly and can then be corrected.
In the quantitative reasoning section, it is useful to sharpen up certain arithmetic skills which you may not have had to use for a number of years. Again, practicing doing these at speed and going over your answers is helpful, particularly given the relatively limited number of basic mathematical functions which you might need to use.
Of all of the sections, abstract reasoning is probably the most important to prepare for. Thinking about the properties of the symbols used in the test, and being confident of how to look for patterns methodically, is key to give yourself the best chance at this difficult section.
In the situational judgement test, you are asked to make nuanced decisions about situations in which there are conflicting ethical, practical, and professional priorities. Some advantage may be gained here by learning some of the principles of medical ethics and good medical practice, and by understanding the reasoning behind sample questions.