Taking a 5th subject won't be of any advantage in applying to medicine, except maybe for Kings. So you can take food tech if you really want, but it's no advantage and might harm your grades in the other subjects? Also, a lot of med schools ask for 'academic' subjects, so food tech wouldn't really count. It is seen as more of a vocational subject, and Imperial (considered a top university) state that they don't count any vocational subjects at all. Something like english, history or a language would look really good, but it depends if you are interested in them enough to do the hard work.