If it does not state on their website/prospectus/admissions document/selection procedure document/FAQs then they probably don't have a specific percentage that the UKCAT (or for UCL and BSMS the BMAT) is worth. Generally, read the info on their websites, the wiki pages on here, then plough through the individual past year threads of Med Schools you are interested in as you can pick up lots of useful information as to previous years cut offs, info given out informally or at open days and stats that have been successful at obtaining interviews/offers. You can also use good old Google and there are lots of other sites with Med School details pulled into one place for wannabe Med Students. If there is a particular Med School you are interested in, and cannot find the information any other way, then you can ask on here, but a blanket "how do Med Schools use aptitude tests?" is quite hard to answer as they all use them differently!