I would not say that the UKCAT is a lot harder, just more unpredictable ie. the revision and preparation you put in isn't rewarded as much as for BMAT......although don't let anyone tell you that it is not worth revising for these tests/paying for resources etc..........this is exactly what I was told and with hindsight it is these same competitors who are buying every book and going to every preparation course.
If your budget permits.....1 revision book and 1 prep course for each (preferably not....any of these budget courses which are here one year, and gone the next) + make the most of all the free resources online
For BMAT, I spent a 1-2hrs/day for 3-4 weeks leading up to the test.....and that worked quite well. Pretty similar amount for UKCAT as well