Also, just if you want some idea of the limits of what *is* possible:
I have two part-time jobs. Between work and uni, I do 7 days a week. I live on the other side of the city, so I deal with about 8 hours travel time to/from uni every week. I'm studying 75% load (3 subjects a semester instead of 4), just so I can pull it off without failing. I don't have as much time as I really need to study, so I'm managing a distinction average at the moment. I succeed at having a social life, but I probably only go out with friends every second or third week (though I do spend plenty of time with friends at uni). I also have a good exercise regime.
Coming from someone who knows how to put in the hard yards, I can safely say you'll be able to manage a "pocket money" job quite easily... all you're after is enough hours for a bit of disposable income. Maintaining such a full-on schedule like mine is very demanding, and not something I recommend someone try to keep up indefinitely. The only reason I've been doing this for the last three years is so I can save up enough money for student exchange next year. I'm working 7 days a week at the moment (and close to full-time hours) simply for one last boost to my savings before I head off. I'm so over it all, and the only reason I'm still doing it is because I know that as soon as I've decided I've completely and absolutely had enough, I can hand in my resignation and walk away.