I think I would include a lot on why you want to do history of medicine specifically (as opposed to just history or something more general), and why you want to go to Imperial, what is it about the course there that interests you more than other universities?
State your aims (if you know them), do you want to continue to phd, a career in research, make sure they know that you aren't just doing this because you don't know what else to do.
Mention some of the skills you have developed at under grad and how you are going to apply them to your masters, talk about anything you have extra-curricular that i kind of relevant but also makes you seem interesting.