I can't speak for undergrads anymore as I 'm back at uni on a reasonable research grant with books, equipment etc. paid for. However, I reckon I can get by on 30 quid a week on groceries (and I like to eat well!). That's on Bristol prices too, which are about as high as you can get outside of London. The job thing is great - as an undergrad I worked about 25 hours a week in my first year, 15 in my second and 10 in my third, plus full time during the holidays. It was a tough slog at times, but well worth it as I was usually too exhausted to go out boozin'. I was lucky - my parents paid my 68 quid a *month* (cheap Belfast prices) rent and I escaped with no student loans at all. That was 7 or so years ago though.
I would suggest that getting food allowance is great, but there is more satisfaction in drinking the money you have earned yourself (and less of a guilt trip about wasting money too).