Hi,
So you went to maths olympics!
(you're talking about la olimpiada matematica espanola right?) it will be a very good experience for you. This way you can learn lots of things out of school syllabus.. which is just perfect to show your enthusiasm in the subject when it comes to the admission.
Well, about your questions:
1º. I think you are talking about Bachelor and Master. Bachelor is a 3 year course and Master is a 4 year course. I don't think the Master means that it covers Postgraduate master's degree ,yet I'm not sure. In the UK, lots of degrees are like to be 3 year course. A subject like law or mathematics which are considered to be very competitive to get into, are 3 three year course as well, so it's not likely to be worse to have Bachelor's degree than to have Master degree. However, in case of engineering (or some other subjects) there are both Bachelor degree and Master degree so I would say 4 year course master degree will be better, and obviously, more competitive to get into.
2º. Well..... if you study in a good university such as Oxbridge or Imperial, you won't have any trouble of getting a good job there. I'm not sure what aspect you want to know about staying in the UK. The career of graduates can be found in each universities' web-site.
3º. Just I like said, you will need to do some extra-curricular things. If you have any chance to do A-levels or IB (International Baccalaureate) I would encourage you to do it as they are more likely useful to get into a British university. If you don't, however, don't worry it doesn't matter. British universities accept Titulo de Bachillerato espanol as well and their typical offer condition can be found in each universities' web-site (there are some universities, like Imperial, that doesn't mention it. In this case you should contact them sending an email and ask about it). I saying you should do extra-curricular things not because they don't like (well they don't prefer it neither) it, but by having a good CV you will be able to stand out. To be honest, I think I got an offer because of my CV (I'm doing IB and Bachillerato together so maybe this might have been good as well)
4º I'm sorry but I don't know anything about it... As I'm looking forward to do Master (postgraduate) and Doctor's degree after finishing my undergraduate studies I practically didn't think about it. However, you might be able to get some information about them in each universities' web-sites.