It's very different for each university, but if you don't meet a cutoff for any factor then your application is instantly rejected.
Every school will have an A level prediction cutoff, this will either be AAA or A*AA, in one case AAB is considered and sometimes an additional AS or A level will be required for the offer, usually a B or C grade.
Some will have a UKCAT or BMAT cutoff, others will rank or score these and a few will use it to discriminate between borderline applicants.
GCSEs have minimum grades which are required, then there are cutoffs which will usually be something like 6A's or just minimum grades. Many uni's will score GCSE's and rank applicants based on these scores too.
Finally personal statements can be given a score which is usually added to other scores, or where a score system isn't used, to discriminate between applicants who meet minimum requirements.