Hi
I've moved this thread to our engineering sub-forum so that you can get advice from current engineering students.
Firstly, you'll need to achieve the best grades you can at AS, if you can get AAAAA that will put you in a very strong position. If it's possible, doing some relevant work experience this summer could help your application. You could also see if any local universities have open lectures etc that you can attend. Most people will also talk about a couple of books/articles in their personal statement, so if you haven't already, you'll need to start doing some extra reading.