Well if you already have the 2:1 it won't help your application to medicine unless you can somehow blag it on the personal statement.
Masters these days are near enough redundant in our field as biomedical scientists; the only genuine reason to do a Masters these days is as a foothold to a PhD if you didn't get a 1st class degree. Many employers will offer the chance to study for a Masters degree (e.g. the clinical scientist programme).
If you're planning on doing research afterwards if you don't get into medicine, and you don't have much lab experience from your degree, it's a good idea as you won't get a PhD without it. If you've got good lab experience from a final year project and a placement, you probably don't need one if you have a 1st class degree, but you might with a 2:1 or lower.