Thomas Crapper's name is inevitably linked with the toilet. "T. Crapper Brass & Co., Ld." was emblazoned on toilets all over England. It was the American soldiers stationed in England during World War I who returned to the US using his name as a euphemism for the toilet. Crapper died in 1910 at the age of 73, although his company lived on until 1966 -- 105 years in business. (However, the company has been resurrected and is again alive and well (see below).
I'd say that in British English it isn't even a mild oath. It's companion beginning with 's' is still a little rude, but 'crap' could be used anywhere outside formal English, IMO. I wouldn't hesitate to use it in front of my grandmother, which is my test for acceptability.