Most people think that, after they've been living with their partner for a couple of years, they become 'common law husband and wife' with the same rights as married couples. This is not the case. In fact, couples who live together have hardly any of the same rights as married couples or civil partners.
There is no such thing as ‘common law marriage’.
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.
I think you mean English common law. There's not such thing as "British law" as such, because Scotland has its own legal system (which incidently is partly based on Roman law).
So maybe they're not that crazy :)