Can someone please explain to me what a 2:1 degree is?
In British universities, degrees are conferred with one of three classes of honours, depending on how good the results are. The best are first-class honours degrees, then come second-class honours degrees, and then are third-class honours degrees. Universities occasionally award degrees without honours to someone who's done well enough to pass but not well enough to graduate with honours. Most degrees are second-class honours degrees, so to distinguish among them, they're divided into two divisions, upper and lower. A 2:1 degree would be the upper division of a second-class honours degree, which is a good result. I think there are more first-class honours degrees given out these days than in my day, but I know in my day only about 5% of degrees were first-class honours degrees, so it's no shame at all for someone to end up with a second-class honours degree; that's what most of them are.
As a general rule, if a person wants to stay on at university to do postgraduate work, a 2:1 degree would be considered good enough for them to do that.
Also, what are A-levels? Sorry, I am American.
"A levels" are exams that are taken at the end of the last year of high school. Again, things have changed since my day, but the exams are part of the General Certificate of Education qualifications. In my day, kids sat O-level exams (Ordinary level of the GCE) at age 16 (I think that would be the sophomore year in high school) and A-levels (Advanced level of the GCE) at 18 (the end of their senior year). At some point the O-levels were merged into another exam and turned into GCSEs instead of GCE O-levels, but the A-levels are still the way they always were.
Depending on aptitude and school policy and so on, people tend to take GCSE exams (or, previously, O-levels) in quite a few subjects - anything from 2 or 3 to 12 or 13 or sometimes even more. These are the exams Princess Diana famously failed all of her attempts at sitting. When they get to the stage of studying for A levels, kids are expected to specialise, and they usually take between 2 and 4 subjects. I don't know if it's still the case, but it used to be a legal requirement to have at least two A-level passes before you could get into university.
Just as an aside, the exams at Hogwarts are based on O-levels and A-levels. O.W.L.'s are Ordinary Wizarding Levels, and are taken at age 16, and N.E.W.T.s are the equivalent of A-levels and are taken in fewer subjects at age 18.