I know it may sound like a cheap plug, but have u tried this book?:
Amazon.co.uk: Passing the UK Clinical Aptitude Test (UKCAT) and BMAT 2008 (Student Guides to University Entrance) (Student Guides to University Entrance): Felicity Taylor, Rosalie Hutton, Glenn Hutton: Books
i found it quite helpful.
Saying that i felt i was terrible at them and still managed a 550 score. But dont worry 450 is NOT terrible.
Practice will help you!
Also have you tried the UKCAT website?
UKCAT Details
Calm down.
Remember u still have plenty of time to practice!
Everyone will be in the same boat as you when u do it!
Hope I helped,
Wavey G.