I completely understand, I will try to answer your question. I also attend a highly competitive college, and you have to get pretty high GCSE results to get in, similar to the college that you are wanting to apply to - one has to get 7s in all subjects that they will be studying.
Do you have mock grades that you can give? As I had low targets like your self, as I didn't do very well in SATs, e.g - I was targeted a 4 in Chemistry and got a 9. I would give the college a sheet of your most recent mock grades. I know that my college offered previsionary places for those that had low targets, and if they did do well on results day they would be granted a place.
Upon looking you would have to get an average grade of 7. So if you were doing 10 GCSEs you would have to get 70 points - all the grades added up. If you work hard, you can do it!
If you need any help, I am more than willing. I do Chemistry, Biology and Geography at A-Level.