I know the Grade's range for Grade 1 - 8 (at eight you're consider a "professional" I believe). But that's about all I'm able to really tell you.
I've done a search in Google, have take a look at this link below and browse around a little, hopefully it should give you all the information you need - it seem's pretty informative from what I've briefly read, so it should at least give you an idea of where to start looking if nothing else.
http://www.registryofguitartutors.co.uk/exams/electric/electric_guitar.htm They also do Bass and Classical guitar, so if you're not after electric, you should be able to find what you want.
Hope that's of some use,
Regards,
Chris.
Don't worry too much about me, ignore me long enough and I'll go away.