Definitely worth working on the English to pull up to a B or better. A C may well count against you at top unis. Having said that, I got a D/C borderline in my English GCSE mock exam, but managed to pull 2 As in lang and lit through hardcore revision and decent coursework grades.
Computer Science is pretty much (as I understand it) the theory of computers at everything from the transistor level to the highest levels. Slightly less applied and probably roughly equal in "respect" than software engineering, which is the same but with more of an emphasis on practical programming.
Personally, I chose Biology over Computing at AS for both a change in style and a change in the sorts of people in my lessons (more females :P). Also, you need to have Biology for biological-type degree studies, but only maths for computer science.
I'm applying for Physics BTW
Hope this helps!
Edit: I did Physics AS, Chemistry AS, Biology AS and Maths A level (in one year) last year, and this year I've dropped to Further Maths A level (in one year), Physics and Chemistry.