The lowest I remember paying was when I moved to another part of the country and it was $.89/gallon. I remember it being just under $1.00 before that, but never in the 80-cent range.
An Imperial gallon is also considerably more than a US gallon, too - 160oz as opposed to 128. The difference between an Imperial ounce and an American one isn't significant, but a British pint is 20oz, an American one is only 16.