It really is quite silly now, to be honest. We have four different grading systems across these nations, and each system has to be understood by employers, FE and HE providers.
So at 16+, England has a 10 point scale of 9-U, Northern Ireland has a 10 point scale of A*-U (with a C* snuck in the sequence), Wales has an 9 point scale of A*-U, and Scotland's N5 has a 5 point scale of A-no award.
Even England can't decide whether a good passing grade is a 4 or a 5, and Northern Ireland's C* suggests a better than normal good pass. I don't have a clue where the comparable line is in Scotland, although the stats would suggest it is somewhere between a B and a C.
Mr Gove's reforms have a lot to answer for!
EDIT: Oh, and I forgot my post script: Independent schools can opt to take IGCSEs under the old English A*-U scale (retained for most purposes by Wales).