The British Rule simply states that if the reigning monarch of the United Kingdom is a woman, then the values of the King cards and Queen cards are reversed.
Since the current reigning monarch of the United Kingdom is Queen Elizabeth II, British Rule contract bridge is currently played with the Queen cards outranking King cards.
The British Rule is used almost exclusively for card games played within the UK, but it is sometimes employed in other Commonwealth countries and crown dependencies, where it is commonly referred to as The Commonwealth Rule.
In the Victorian era (when the reigning monarch was Queen Victoria), The British Rule was widely used in card games throughout the British Empire. In certain parts of India, local bylaws enforced its usage, punishable by a fine.
However, in the modern era, the use of the British Rule has become entirely optional. It is most commonly employed in private members' clubs and bridge and golf clubs that maintain a strong traditional ethos. For international tournaments played within the UK, the British Rule does not apply.
The British Rule is almost never used in games of poker. However, the rule is sometimes suggested by players as a way of unsettling opponents who are unused to the different priority of the cards.