Well, in the exams key terminology needs to be used to get the marks. There is quite a large volume of content, though if you can succeed in History you should in Bio! Is is possible for you to have a taster session or look at the textbook?
As for the A, if you work hard - and work smart - it is possible. Sometimes people just cannot grasp a subject but that is pretty rare, hard work is what the A-graders put in.
You mention that you'd only have 'one traditional subject' -
what do you think you'd like to do at uni? If it's History, you could do Eng Lit or a language instead of Bio. If it's Pysch, then Bio would be useful. If it's the P&E then you could do Eng Lit/ a language again, or another essay subject.
I suppose the question is, why do you want to do Biology? If it's mainly to have an academic subject, then don't. There are many other options. Biology at my college is the one science the most people dropped, because it's the one science that is most often not taken with another science, and those who prefer essays who did it found it a lot trickier than they expected. It's the grades not the subjects (after you meet the subject requirements of course) that matter!
BUT if you do enjoy it you should try it at least, for fans of biology it's a very interesting course!