(from Cambridge International Dictionary of English)

bog (TOILET)
noun [C]
BRITISH AND AUSTRALIAN SLANG
a toilet

I'm just going to nip to the bog.

We've run out of bog paper/roll.

I was on to this slang term because I've been publishing in an Anglo-American journal called Bogg for the past several years that claims the same slang connotation for it's name, though I can't find a citation for the double "gg" anywhere. There's no current web page, but here's a review of Bogg and the journal's background info:

http://www.nhi.clara.net/mg0028.htm