To answer your question it depends on the role. However for intelligence officer if i remember correctly, you need a 2:2 degree, be over 21, a british citizen, not have used illegal drugs in the past 12 months. There would also be a security and background checks for any criminal convictions. It would also be an advantage to know a foreign language.
Oh and take it from me (if you've missed what everyone else has already said) it's not as exciting as it seems. You will most likely be sitting at a desk 8 hours a day or shadowing a diplomat and going to diplomatic conferences. There are james bond-esque (i use this term lightly) roles in the SIS but you don't apply for them, you are chosen to do them and they are few and far between.
If you want adventure, join the militay or even better join the police. You will never be short of action in the police and funnily enough they are more like the films than the sis.
Also you aren't supposed to tell people you've applied, after all it is the secret intelligence service.