I'd either spent it on a new bicycle, or save it for when parts on my bicycle fail.
My flatmates are the same as the OP, no interest in saving, they moan at me sometimes for having a large amount of money saved and that I refuse to spend big on nights out, I usually look for the best value for money in what I drink and eat or anything I buy. What I tell them is, I never know when something will crop up that I need the money for.
Good job I saved, my rent went from 82 a week to 94, with the money I saved I was no worse off than the year before. One of my bicycle wheels failed from wear and tear after a year of use, probably 12,000 miles on it maybe more, so I needed a brand new wheel set, that was 175 quid. I then wrote a bicycle off in the summer, I never seen that coming...
Then my flat mates try organise crimbo dinner, the person who is going to order it on their card finds they suddenly have no monies left so they all go buy takeaways instead.