Only around 10 to 15% of students in the UK - perhaps even less in law, certainly at my law school - would get above a 2.1 so it's pretty high. I don't want to get into a whole debate on what I'm going to say next, because people can argue on this point endlessly, but at some universities (I can point to the two in my neck of the woods) there are differences in standard needed for a 2.1. I would say that if Leiden asked UK students for a 1st instead of a 2.1 they wouldn't get too many.
By "2.1" are you meaning second class honours, first division?
I was thinking of the GPA system we use here where a C- (50-54%) is a 1 and an A+ (90-100%) is a 9.
Yes, an upper second class degree. It's nigh on impossible to get higher than 80% in exams here so maybe the systems aren't really comparable in terms of percentage grades etc... who knows how they pick people!