I have an offer for September 2017 and I can honestly say, at the moment, they don't really look at GCSE grades - Cambridge understand that you can change a lot between GCSE and A-Levels and so currently don't refer to GCSEs (unlike Oxford, which prioritise it a lot) in the admissions process very much - it's very heavily focused on interview and admissions tests, alongside things like the personal statement, and of course predicted grades for A level. It's absolutely worth applying if you get the predictions you need for your A levels, regardless of what you get in your GCSEs.
However, this is worth keeping an eye on as with the new reformed A-levels the admissions system may well change in the next few years as people won't be taking AS levels so predictions will be less reliable, meaning they may look at GCSEs slightly more. This is incredibly unlikely though as an admissions talk I went to stressed that this is not a direction Cambridge wants to go in as they don't want to lose out on the best students who have only recently reached their potential in sixth form, and so did not get best grades at GCSE.
It's also worth looking very closely at individual colleges, as whilst this won't change your chances of getting in, it might change how you feel about Cambridge as an option. I looked around some colleges which I found really intimidating, so wasn't sure if I wanted to apply. I then found out about Fitzwilliam, which is a much more relaxed environment so is perfect for me - there really is a college for everyone, so even if you're not sure if Cambridge is for you, you will definitely find something to suit you (even if it takes a little while to find).
I would also recommend retaking your English GCSE if possible, as long as it won't affect your other grades, as even if it won't affect you getting in to uni, some employers have minimum grade requirements for maths and english and won't recruit you if you're competing against another candidate with a higher english grade. If you really want to concentrate on your a-levels, perhaps you could take a gap year and retake your english then?