Britain is a parliamentary democracy with a constitutional monarch. One time, the monarch, had real power but gradually, over the centuries, more and more of this power, has been transferred to parliament. In the XVII century there was a civil war between the force of the king and the parliament. In 1649 King Charles I was executed and the country became a republic for 11 years. The monarchy was then restored but the parliament had become the supreme authority.
At first only comparatively rich people were represent in parliament but during XIX century the right to vote was gradually extended to all man, and, in 1928, to women too.
Britain does not have a written constitution. The division of power is based on laws and traditions which have devoloped over the centuries