i think it should just be said that studying for a different first degree is a terrible idea if your primary goal is to become a doctor, even if it's in a related discipline.
in doing this you use up your entitlement to funding for an undergraduate degree, therefore ruling out a second 5 year medical degree unless you have significant family support. you would need to pay 9k every year in fees. this leaves you the option of GEM, for which entry is even more competitive.
the UKCAT cut-offs for GEM courses are higher than any 5 year course. to get an interview at Warwick you need to be in the top 10% of test-takers. so you'll just prepare really hard and everything will be OK? unfortunately, both UKCAT and the GAMSAT are essentially IQ tests so there is only so much you can do to improve your score. if you're scoring around the median on your first go, you are unlikely to ever get a score that will get you an interview. if you weren't good enough to get a 5 year place, a GEM place is an even less likely. do you want to gamble on being an exception?
i also would like to note that GEM is graduate entry not a postgraduate course. it is an accelerated 5 year course, and does not assume much if any prior learning. therefore, an undergrad in biomed is serious overkill as preparation, certainly not worth jeopardising your chances of a medical career altogether.