I don't think you can ask people to decide between wanting to go down an academic route and get a degree, or to opt for a more vocational path, especially aged 16. The whole point of A-Levels isn't about university entrance, it's about saying that people with these qualifications are this well educated, hence A-Levels are stand-alone qualifications.
On the other hand, I think oxbridge entrance tests have been proven not to work, even less than interviews.
I do get the sense (and it is only that) that university admissions are moving more towards the subject testing BMAT/LNAT exam idea, especially towards the top end of the spectrum. I personally can't see why such things can't be incorporated into STEP papers or AEAs. Although, if the A-level was made more complex, it would alienate less academically-inclined students.
A synthesis of the scottish system, whereby there are different levels of exam, each of which acts as the entrance exam to the course above it, and the english system of modular-based A-levels might work; certainly the government should be, and I'm sure are, investigating these types of things.
On a different point:
In state schools, by and large, it is much easier to go off the rails and not focus academically than at private/selective schools, mainly because of exposure to "the other half" of people who don't care about uni etc. A big reason comprehensive kids go on to go comparatively further at uni is they've learned how to deal with lots of different pressures, and they're going to be very academically focused (if they weren't, they'd just have done what their friends did). It's less of a deal at selective schools because there's a whole load of problems taken right out the equation.
That's just conjecture, tbh, but I think it stands up to reason!