Whilst you can certainly guarantee an interview at some places with a very high UKCat score, by no means can you be admitted without a decent personal statement/interview.
Actually, speaking to the admissions guy at Barts and The London, they only look at UKCat. Personal statement comes into play for the interview, but isn't particularly key for gaining that interview.
In addition to all this.. you need to have passed the first stage of the application process by achieving the required GCSE / A-Level grades or predicted grades for any university to start considering UKCAT.