Why do the British use "bog" to refer to a toilet ?

I encountered this sentence while reading P.D. James' The Skull Beneath The Skin:

"You'll be beholden to Mr. Gorringe for nothing but his light and heating and the water that flushes his bog."

A quick search this morning finds "bog-roll" for toilet paper (although a "giant bog roll" apparently refers to paper kitchen towels), and "bog standard" to mean ordinary or average:

But, why 'bog standard'? It may be the association with the word bog, which has long been used in the UK to mean toilet... The other most often-repeated theory of the derivation is that it is a mispronunciation of 'box standard', the term referring to unmodified goods coming straight from the box.

There is some discussion re possible etymology of bog=toilet at The Phrase Finder, but I haven't found a satisfactory answer.



The book, BTW, is a classic English-country-house-murder mystery, set in a castle on an island where a half dozen people have reasons to be pleased about the victim's death.  The detective is Cordelia Gray (not James' iconic Adam Dalgliesh).  I rated the book 3+ ("pretty good"), not the 4+ required to make the "recommended books" category in the sidebar, so this post gets filed only under "English language."