Hi there! Congratulations with your AS results!
I actually managed straight As at AS (and A*A*A at A2), but you definitely shouldn't let the B in chemistry put you off applying. I think a lot of prospectives think everyone at Imperial came out with perfect grades, but that's not the case. A lot of my friends had a B in their AS grades and managed to get in for engineering (which is supposed to be way more competitive than life sciences), and there are plenty of people in my year who got a B in AS chemistry.
If your actual mark is only a few UMS off the A, it's may be worth asking your chemistry teacher to mention it in their reference, but either way the B shouldn't be a problem
Aside from the usual (learn everything, practice writing essays, look at past papers), I would say diagrams can make a huge difference. I personally hate drawing in exams, but adding a good, well labelled, relevant diagram into your essay can pull you up a grade.
Bringing in material from outside sources isn't really necessary in first year (I have a friend that did no reading whatsoever and still managed to do better than me
), but it can make the difference between a low first and a higher first. If you find a particular topic really interesting, read around a bit, and add it to your exam answers (If it's relevant! Don't put irrelevant reading material into essays just to show that you've been reading, because that will actually pull your marks down). But definitely don't feel like you have to do lots of addition reading after each lecture to get a first.
When you get closer to exam time, it's worth asking some students from the year above about their experiences with different lecturers. Some lecturers are nicer markers than others, and hence some students do like to play tactical when it comes to picking which essays to answer (you get a choice of question in the exams).
In terms of prior reading...I would honestly say "don't bother". I actually got quite worked up about reviewing A level material before the start of uni, and it was a complete waste of time. One of the topics started off with completely new content, and the other reviewed some of the old chemistry, but at quite a slow pace (almost as if they expect you to forget everything over summer
).
If you're adamant about reading ahead I can give you some book names and topics, but they work you very hard anyway once the year starts, so I would suggest you enjoy your Summer before that kicks off.