If you're likely to do all those extra things anyway, then go for the school that has a better atmosphere or is more convenient to get to - depending on your priorities. I have to say, getting up at 5am doesn't sound particularly appealing to me (and having voluntarily got up at similarly ridiculous times last year for rowing will say that the lack of sleep can really get to you if you're not the kind of person who goes to bed at 9pm)...
Ok, the volunteering doesn't sound quite so bad then - my school had a similar policy where you had to do at least one activity/ volunteering each week. Since you'll probably want to do something like this anyway it's not that bad. I'd recommend picking activities which are relevent to the degree - e.g. I joined the Chemistry club (sounds a bit nerdy, but was really good fun...) and volunteered to help younger students at lunchtime workshop sessions
Things like the Maths Challenge/ Olympiad is definitely good for your uni application - particularly if you get through to later rounds. There are similar Olympiads for all science subjects (Biology, Chemistry, Physics), the Cambridge Chemistry Challenge, The National Cipher Challenge, etc. too so ask your school if they are offering those if you're keen (if you ask early enough, they may be willing to register even if they hadn't previously offered them).
An EPQ is great if it's on a relevant topic, make sure you give yourself plenty of time to work on it - I would recommend trying to get a lot of it done over the summer between AS and A2 if possible.
Since you're interested in computer science, learning some programming languages now would be really helpful - the Cipher Challenge I mentioned earlier is a great way to practice Python, and you can do things like creating your own website too
Running a blog writing about interesting topics in maths/ CompSci is a great way to demonstrate a long-term interest in the subjects.
It's also a good idea to read a couple of relevant books - if you look around your school/ local library there should be a popular science section, where maths and Computer Science will almost certainly be well represented (along with Biology, although I have always been disappointed at the lack of interesting Chemistry popular science books
). Most students will mention a couple of books in their personal statement, and if you really want you can read a few undergraduate level textbooks to demonstrate you are genuinely interested in studying the subject at uni.
You should also consider applying for summer schools and taster days - for example the Headstart, UNIQ and Sutton Trust summer schools (all one week courses in the summer holidays between AS and A2), and I think some of the Smallpiece Trust courses too (you may even be able to do one of those this summer, last time I looked they were advertising places left on ones for yr 11 students). There are London, Cambridge and Oxford taster days too - the London ones are held at random times throughout the year (I think they also announced some new ones recently too - google it), whereas the Cambridge and Oxford ones I think are after Christmas/ around Easter time (although they fill up REALLY quickly, so sign up for an email reminder to apply as soon as applications open). Summer schools and taster days are usually free or heavily subsidised, and some prioritise students from particular backgrounds. If you don't meet their widening participation criteria, you may be unlikely to get a place on the more competitive ones - but if you are able to you could go on a less 'outreach'-y course which would probably require more money (but usually not ridiculously so). Try and avoid any courses that promote themselves as Oxbridge admissions workshops or seem a bit dodgy - these tend to just try and get loads of money off you for no real benefit.
You could also consider applying for a Nuffield Bursary project - these generally take up most of your summer between AS and A2, but allow you to spend time at a university getting involved in their research (and you may get paid). This would be a fantastic experience - not just from a getting into uni point of view, but also work experience for when it comes to applying for internships/ graduate jobs. Many of the projects are more science focused, although there are some maths ones, but there are often a lot of opportunities to get involved in the computing side of them which seems pretty good for you
Applications open pretty early (I think around Christmas time) and are reasonably competitive, so look into this soon.
Hope some of this helps
I'm currently working on a blog,
www.passionateaboutscience.co.uk, which promotes outreach opportunities etc. which you may find interesting - it hasn't been updated in a while unfortunately (exam revision...) but there's a pretty useful summary document listing annual opportunities which you may find useful