There's plenty of practice material for the UKCAT; it's a very typical aptitude test. You know for sure that Abstract Reasoning will come, Quantitative Reasoning, etc.
You never know what will come up in the BMAT. Therefore I'd say the BMAT is harder than the UKCAT.
Section 2 of the BMAT is very basic, not very difficult. It's section 1 most people have a problem with. Then again, section 1 is very like the typical aptitude test.