OK, I'm not trying to be inflammatory, but I do think that is the best thing for you to be doing (
)
You
could do your SII now, but what on earth for? The way I see it, the first hurlde you have is getting into a top uni, getting a job comes
after that. So, you should be trying to do whatever it is that will make your Uni application stronger, not your CV for an interviewer - do some voluntary work, get an
interesting job for the summer - maybe even try and work abroad????
IMO you have a better chance of getting the type of job you want if you jump through the hoops. Academic Excellence is a pre-requisite for the uni's you want to be looking at - and to get the best grades you can in your A'levels, you really need to be taking a break and having some fun over summer. It might sound a bit "mystic meg", but it's the truth. You are too young to be putting yourself through some regime that'll stress you out - and probably impair your future performance in college.
Instead, it would be better to spend your time doing something you enjoy doing, and will give you something to talk about when it comes to Uni interviews - D of E, voluntary work, travelling, and so on... when push comes to shove, there will inevitably be some chinese kid who has twice as many qualifications as you do, so you've got to give yourself something else.
Do all that, get into Uni,
then think about your CV.