Having attempted the UKCAT last year, I can honestly say that this book is the one that is the closest to what you will experience on the day of the test in terms of difficulty of the questions. It offers a massive range of questions, all backed up with good reasoning and tips on how to answer quickly. The presence of mock exam at the end of the book makes it one of the rare books on the market to offer the complete package and, most likely, the only one you will really need to pass the test at the level you need, whatever school you are applying for.
I would also widely recommend another book written by ISC Medical: Medical School Interviews: A Practical Guide to Help You Get That Place at Medical School - Over 150 Questions Analysed
, which is one of the most fantastic and complete book that you could ever find on medical school interviews.