Most medicine unis give the opportunity to intercalate now. Though not all are able to.
Basically you are taken a year out of medicine to study another subject and get a degree in that. You are pretty open to choosing to intercalate in anything, hypothetically. In unis where it isn't compulsory they will have limited options as to what you can study. At my uni you are pretty open as to what you intercalate in, you are given a list of options but I'm pretty sure if you make a good case as to why you want to study something else and how it relates to medicine you can do it