I didn't work during term time at Uni. I was at Durham and terms are shorter than most so you get more lectures "crammed" into a shorter time period. I was doing a science degree though so I had 2 days of labs and then 3 days of virtually full lectures (although having been working for a number of years I long to have such a "crammed" timetable.) It may be easier if you are doing an arts degree with less contact hours.
I think I would have struggled to work in addition to what I was doing. If I did have a regular job it would have been at the detriment of any extra curricular activities. I had planty of friends who regularly had to miss out on trips and competitions because of work. Of course I was actually doing something. I was paddling (kayaking) a lot, I competed in polo, slalom and river race. In my final years I was acting as a river leader so coaching and looking after new paddlers, I was helping run the club etc. We ran Wednesday coaching, sunday night pool sessions, saturday river trips, tuesday night fitness, thursday night socials. I also had a sideline interest in mountain biking and ballroom dancing.
I did a masters degree and my 4th year was a bit more challenging. I spent a lot of time in the labs but also, because of the experiments I was running I would often have to be in relatively early and stay relatively late (with a 3 hour lunch spent riding in summer).
Work would have been valuable experience but I think I got other useful experiences and "life lessons" from extra curricular activities. Who knows when an ability to tango or quickstep could come in useful! You've got plenty of time to get used to a world of work, university can be fun and you can learn a lot while having fun. I've always had a reasonable work ethic, probably learnt more when I was a lot younger through my parents...
In terms of jobs it is most worthwhile looking for summer internships. This kind of relevant work experience is very highly valued by employers, probably more so than bar work.