It does vary, but I got very little help from contacting the universities themselves. My qualifications are pretty specific: I had a HS diploma from a "gifted school", some seriously crappy grades (like a 2.0) from my first go at college, and an AP US History score of 4, an SAT score of 1330, 2.5 years experience as a legal secretary with a few courses provided by my firm, and... that's kind of it.
None of that matched what the requirements for the Uni of Manchester law programme and i got in anyway. I did NOT get accepted to Leeds, but I did to University of Manc, Man Met, and University of Westminster in London.
I didn't hold out much hope, but I was pleasantly surprised. I eventually gave up on the universities and just filled out the UCAS application and crossed my fingers. It can't hurt to try to call them, but don't get too downhearted if they don't help you as much as you'd like. I put up my experience to show the haphazard qualifications that got me in. If you have any questions, just PM me!