I'm 16 years old, and really like electronics - I plan to study EE at university (it wouldn't be Oxford/Cambridge, as they don't do EE, just general engineering),
Take a look at Southampton University. I graduated there 40 years ago, and when I went back to see an open day last year I found they had the same ethos. Basically both theory and practice are equally important: practice without theory is blind fumbling, theory without practice is merely mental games.
No doubt there are still other good unis (think Russell Group), but there are many bad ones and many bad courses. Avoid those that trap you into an overly narrow specialism too early, e.g. telecoms engineering - you want one that gives you a solid grounding in a wide range of topics. You can and will specialise on the job. Speedread this old Isaac Asimov story, since it is as true now as in the 50s:
http://www.abelard.org/asimov.phpHaving said that, if I was starting again I'd look at biochemistry and the life sciences since that is at the same stage as electronics was when I was your age. I'd also look more carefully at the Cambridge colleges for the reasons others have mentioned.
I'm looking for some guidance for how much revision I should be doing for my GCSE exams coming up in May/June.
That's the wrong question since the answer is simple and unhelpful: you should do the right amount. Too much and you burn out, too little and you foul up in the exam.
A better question is "how do I know when
I've done the right amount of revision?". The technique which worked
for me was to spend one/two hours reading my course notes a page/section at a time, then closing the notes and validating I could remember everything in that passage. Once I could remember everything that was on the page and why it was there, I moved onto the next page. Of course it is easy to fool yourself that you know something; you have to be brutally critical and honest with yourself!
When I got stale after one/two hours, I went and did something completely different to relax. In my case it was walking the dog. In my daughters case it was learning to fly a glider (you are already old enough to be a solo pilot, provided you demonstrate you are safe). The activities associated with gliding were
invaluable to my daughter and made her much more employable in
any field. She could
demonstrate (i.e. not merely claim) that she worked on her own and in a team, worked in a safety critical environment where she was responsible for other people, and that she could react appropriately when things went wrong. She didn't realise how much it had helped her, until she went to uni and realised how much other people
hadn't done. Plus it is great fun

I've been told that when it comes to EE, experience is much more important than exam results for employers, but to what extent is that true?
For the good jobs, it is false.
Crude but useful analogy: doctors know the theory of making people well and nurses know the practicality of making them well. If I want a diagnosis and course of treatment, I want a doctor not a nurse. If I want blood taken or a fracture plastered, I want a nurse not a doctor. The engineering equivalents are engineers and technicians; both are
necessary, neither are
sufficient, and vive la difference.
However, you will impress employers if you can decide on a project that will stretch your understanding and capabilities, complete the project, and be able to say what you would do better next time. Summary: accurately assess theory practice and your capabilities, predict where the dragons lie and avoid them.
If I have 2 hours a day to spend on either revision or electronics, which should I choose to make me most employable?
At this stage concentrate on revision. But by all means use playing with electronics as a relaxation.
It may be possible to get local advice from a mentor. Contact the local branch of the IET via
https://www.theiet.org/