|No commercial preparation courses are endorsed by the UKCAT and on page 26 of the official guide on the website states that "[candidates] should be sceptical regarding the claims made regarding UKCAT coaching".
Books and practice questions are acknowledged to be helpful, with the UKCAT official guide cautioning that only recently published or updated resources should be used. UKCAT research also suggests that around 25-30 hours of preparation leads to higher test scores. 
Free practice tests which closely mimic the live test are available on the UKCAT website here.
Substantial question banks, with over 1,000 example questions are also available on the UKCAT website here.
The UKCAT website also provides numerous other free resources to help students prepare for the test, include detailed tutorials on the test, explaining each individual sub-type, a free App students can download, advice pages and tips from past candidates, and the Official UKCAT Guide. These are all available here.
Familiarising yourself with the styles of questions has been found to be useful and some people recommend getting hold of some old 11+ style test papers for practice. Practice questions for each section of the UKCAT can be found with each section.
Thanks goes to Lu-x (unless otherwise credited) for finding and collecting them all together.
The UKCAT book, 'Get into Medicine - 600 UKCAT Practice Questions', is highly recommended for practice questions - 'Get into Medicine - 600 UKCAT Practice Questions' can be found here. This book has been well regarded and used by students over a number of years, but being published in 2009 it might not be the best choice for the newer question styles.
This UKCAT book is a more up to date book with more questions published by UniAdmissions in 2017 . It is used by lots of students, has good Amazon reviews and covers all question types including the new styles of abstract reasoning and the new decision making section.
Another up-to-date UKCAT book used by students published by Medic Portal in 2015  also covers all question types and has good amazon reviews.
The Medic Portal has specialist resources and courses designed specifically to help students master the UKCAT, and boost their chances of getting into top universities. If you sign up to the website, you instantly gain access to 50+ free UKCAT questions with worked answers. You can also subscribe to access around 2000 more. Additionally, they've got plenty of free information explaining every section of the UKCAT, and how the test is used by universities.
The Medic Portal also offers 100% recommended UKCAT Courses created by doctors in cities all over the UK. Click here to learn more about their Mastering the UKCAT course. This course comes with their best-selling Mastering The UKCAT book. It covers every section, focusing on theory as well as practice questions. It can be bought here on Amazon.
BlackStone Tutors specialise in UKCAT and BMAT preparation services. Their intensive courses are provided by current doctors and medical students. A selection of free preparation resources are available on their website. Free Resources
JobTestPrep.co.uk's website has free UKCAT practice materials as well as over 1,000 UKCAT questions, explanations, solving strategies and video tutorials. They offer live and Skype-based courses and tuition services. They also invite users to take part in UKCAT competitions, where they answer challenging questions with the possibility of winning prizes. - 'Their free UKCAT content can be found here'
AssessmentCentreHQ is a very useful site that has tons of free tests and 'success guides' for all aspects of the UKCAT test. Their numerical reasoning section, in particular, is excellent. Their free UKCAT guide has tons of free practice tests and tips etc: 'www.assessmentcentrehq.com/ukcat-practice-test'
GetIntoMedicine.co.uk is a site run by current medical students and features hundreds of free UKCAT practice questions with detailed answers, in addition to tutorials for each subtest - 'GetIntoMedicine.co.uk' can be found here
Kaplan Test Prep’s website offers a number of free resources for the UKCAT, including; a full-length practice test, practice questions and seminars. Kaplan also offer a range of courses to cater for different preparation needs, all of which are updated to include the 2013 test changes.- 'Kaplan' can be found here
Medify is a website that has various free resources for UKCAT preparation and practice, and courses from 1 week to 3 months can be bought. New from 2012 and was highly successful - 'Medify' can be found here
Medic Mind Book 6 UKCAT ebooks which have over 400 pages worth of theory and questions £15 'Medic Mind Book can be found here'
Another highly recommended book, 'How to pass the UKCAT and BMAT' - can be found here, can be found to be effectively when coupled with the free example questions on the UKCAT website.
Whilst these books are readily available online for purchase, it is worth checking your local library as they may be available for borrowing although you are advised that you may have to wait a while in order to borrow them.
In addition to the aforementioned books, there are also a number of intensive courses - probably the most highly recommended in terms of quality and value being the UKCAT Crash Course. It's £129 for the day, which is cheaper than Kaplan and UniAdmissions.
UniAdmissions is a tuition provider and offers intensive courses and individual tuition on UKCAT questions answering strategy. Claims to have 97% student satisfaction rate and 83% application success rate. [ https://www.uniadmissions.co.uk/ukcat/]
Kaplan is a tuition provider offering intensive courses and individual tuition. Claims to have 65 years of teaching experience for the test 
Medic Mind Classroom Course -can be found here Medic Mind offers the most affordable UKCAT course in the UK at £35. Includes free Medic Mind Book and Online Video Course.
Medic Mind Mentoring Program - can be found here. Be mentored for 6 weeks until your UKCAT exam and receive 24/7 support from your mentor where you can ask as many questions as you like.
- A 1-1 mentoring day at UCL
- 1 Month Medify membership
- Money-back Guarantee
- Weekly progress checks
This looks at how logically people can think about written information and whether they can arrive at a well reasoned conclusion.
You are given a passage of text to read and then presented with 4 separate statements. Your task is simply to judge whether or not the statement follows logically from information presented in the passage. The choices are 'true' if it does follow on logically, 'false' if it doesn't and 'can't tell' if you can't tell from the information you've been given.
Medic Mind sample verbal reasoning can be found here
The Official UKCAT's Question Bank - VR questions from the test provider which are of equivalent standard to those you will experience in the actual test.
The Medic Portal's UKCAT Question Bank - Click on the Verbal Reasoning section to subscribe for hundreds of practice questions.
Verbal Reasoning Mini-Mock Exam and Answers
'Get into Medicine - 600 UKCAT Practice Questions'
http://jung.test.typologycentral.com/ - This free personality test uniquely combines two powerful systems for evaluating personality type – Jungian Cognitive Functions and Personality Dichotomies.
http://www.assessmentcentrehq.com/psychometric-tests/verbal-reasoning/ - Excellent verbal reasoning practice page with lots of examples and tips.
http://www.jobtestprep.co.uk/freeverbal.aspx - Free Verbal Reasoning questions
http://www.getintomedicine.co.uk - Free questions
http://ukcatpracticeonline.com/homepagelinks/SampleVerbalReasoning.html (thanks to thecutiepie for posting)
The Decision Making subtest tests your ability to apply logic to reach a decision or conclusion, evaluate arguments and analyse statistical information.
You have to answer 29 items associated with text, charts, tables, graphs or diagrams.
The Official UKCAT's Question Bank - DM questions from the test provider which are of equivalent standard to those you will experience in the actual test.
This assesses your ability to solve numerical problems. You will need to solve problems by using information from tables and graphs. You need some background knowledge (maths) for this part of the test, but only GCSE level.
You're presented with 36 questions associated with a variety of tables/charts/graphs. A basic calculator is provided within the test system.
The Official UKCAT's Question Bank - QR questions from the test provider which are of equivalent standard to those you will experience in the actual test.
The Medic Portal's UKCAT Question Bank - Click on the Quantitative Reasoning section to subscribe for hundreds of practice questions.
Quantitative Reasoning Mini-Mock Exam and Answers
http://www.assessmentcentrehq.com/numerical-reasoning-practice-test/ - Free numerical reasoning practice tests and resource hub, very useful.
http://www.jobtestprep.co.uk/freenumerical.aspx - Free Numerical Reasoning tests
http://ukcatpracticeonline.com/homepagelinks/SampleQuantitativeReasoning.html (thanks to thecutiepie for posting)
http://www.aptitudetestsonline.com/quizzes/freenumerical.asp (password: demo)
This assesses your ability to identify patterns amongst abstract shapes where irrelevant and distracting material may lead to incorrect conclusions.
You will see all of these 4 different question types in this subtest:
For type 1, you will be presented with two sets of shapes labelled “Set A” and “Set B”. You will be given a test shape and asked to decide whether the test shape belongs to Set A, Set B, or Neither.
For type 2, you will be presented with a series of shapes. You will be asked to select the next shape in the series.
For type 3, you will be presented with a statement, involving a group of shapes. You will be asked to determine which shape completes the statement.
For type 4, you will be presented with two sets of shapes labelled “Set A” and “Set B”. You will be asked to select which of the four response options belongs to Set A or Set B.
The Official UKCAT's Question Bank - AR questions from the test provider which are of equivalent standard to those you will experience in the actual test.
The Medic Portal's UKCAT Question Bank - Click on the Abstract Reasoning section to subscribe for hundreds of practice questions.
Abstract Reasoning Mini-Mock exam and Answers
'Get into Medicine - 600 UKCAT Practice Questions'
http://ukcatpracticeonline.com/homepagelinks/SampleAbstractReasoning.html (thanks to thecutiepie for posting)
http://www.practicetests.co.uk/Launchtest.asp# (email/name required)
http://discovery.axiomsoftware.com/abstract/abstract.php (email/name required)
Situational Judgement Test
The situational judgement test (SJT) measures your capacity to understand real world situations and to identify critical factors and appropriate behaviour in dealing with them. The test consists of a series of scenarios for which you will need to consider either the appropriateness of possible actions, or the importance of possible considerations.
The questions do not require medical or procedural knowledge.
This section is scored differently to the remainder of the test, scores for the SJT are expressed in one of four bands, with band 1 being the highest.
The Official UKCAT's Question Bank - SJT questions from the test provider which are of equivalent standard to those you will experience in the actual test.