They tend to primarily look at your grades, then your personal statement, which needn't necessarily refer to specific extracurricular activity as long as you can demonstrate and interest in and commitment to the subject otherwise (e.g. by reading around and discussing this). After this, they may explore your motivations somewhat in the interview but as I understand it's primarily technical. Extensive extracurricular activity, either within or outside of your subject area, while appreciated, isn't expected as they understand not all students have the same facilities and privileges afforded to them in this regard (that said if you're at Eton and don't do anything except your exams and just get the minimum grades stated on their website they're likely to look down on that).
You may wish to look to see if there are any outreach events offered by nearby universities or tech companies which are relevant that you could attend. Another thing to look into would be various Raspberry Pi events; they often try to forge links with schools and it's possible you may be able to form a Raspberry Pi club or similar and get experience in programming and resources from the manufacturers or other places (or even just buying them after doing some bake sale type fundraising or what have you
). There are also a lot of related but varied "hackathons" which are programming events where individuals or teams program various things acontest or collaborative project.
That said, they tend not to require much in this sense as they can teach anyone programming (and anyone can teach themselves, to a point). They are more interested in you being inquisitive about the subject area and can demonstrate an understanding of the requirements of a CS degree and what it may go on to. More than that however, as indicated, they are much more in your actual technical ability in mathematics to ensure you can cope with the demands of their course.
This seems to be a common thread for Oxbridge applications; unlike their Ivy league cousins, Oxbridge (and all UK universities generally) are recruiting students to specific programmes and want them to be excellent in the relevant areas. They don't care if you play 6 instruments and are applying to maths; they care if you can do maths. Ivy league applications
(actually all US colleges, with the occasional exception of some niche ones, and often engineering departments who try to recruit directly from high school so they can take all the required classes in time) however are just to the institution in general and they don't specialise in a specific subject until their second or third year of four. Thus they look for "well rounded" applicants who have excelled in a variety of areas so they can cope with anything they pick, as otherwise the university could be "stuck" with an student who is excellent in one area but refuses to major in it and struggles in another, who they would be obligated to support to a point.