The UK system uses the notation of 1st, 2.1, 2.2, 3rd as degree classifications. Most individual modules/classes are grade on either an A, B, C, ... scale or a percentage scale. I believe the conversion of A-=70=1st and B-=60=2.1 is pretty universal. At reasonable universities, approximately 10% of students get a first and 60% get a 2.1.
Under no circumstances would I advise converting your own marks on an application. State the marks you got and if it is a particularly bizarre system provide a link where they can get more information.