My allowance last year was £68 per week minus rent, and I always managed to keep my costs below £30, usually closer to £15-20 on food alone. Even managed approx. £10 on food on some weeks, but it depends on your cooking style and how good you are at finding cheap products. That, and whether or not you're the sort of person who buys snacks/fizzy drinks.
Honestly, £55 is probably more than enough. Obviously, it depends on the cost of living at your city, but if you shop smart and try to hunt out clubs and venues that have lower entry fees you should be all good. That, and try not to go shopping for new clothes, etc. every week - a girl in my old flat had the exact same amount of money I did, yet found herself in her overdraft multiple times in the year, whereas I've managed to finish my first year with more money than I started with. You might want to reclassify some things as a "treat" that you need to save up for.