Earlier quoted context omitted.
British law != UK current law but those laws that in the distant past were direct descendents of UK law at the time. British law vs say Roman law. Because of the widespread nature of the British empire remnants of these laws (and remnants of the Empire, such as the Queen being formally the supreme being in Canada) are still found all over the world, even if a lot of those laws have been abolished in the UK itself.
Ah ok, I read that as "Those crazy Brits still have this" and waded in to correct ;)
http://en.wikipedia.org/wiki/Common-law_marriage
So maybe they're not that crazy :)