If you're interested in illustration, you should SERIOUSLY consider an art foundation year; there are lots. You will actually do a "rotation" in illustration in this (well, at most; since this is your end goal you'd be certain to pick a course where you did) and most likely photography, and then you would spend the second half of the course on your "major" area, which would presumably be illustration in your case.
Regardless, even if you apply directly out of school doing photography at A-level isn't likely to improve or weaken your application outside of potentially what grade you get. Therefore, you'd probably be best to drop it. There's nothing to say you can't use any work you've done so far in your portfolio, and it may be an experience you can talk about in an interview (if you can spin it positively about what you learned that you don't want to pursue and what skills you developed that you can translate otherwise).