is a constitutional monarchy. It means that the sovereign reigns but does not rule.
Britain does not have a written constitution, but a set of laws.
Parliament is the most important authority in Britain. Technically Parliament is made up of three parts: the Monarch, the House of Lords; and the House of Commons. In reality the House of Commons is the only one of the three which has true power.
The monarch serves formally as head of state. But the monarch is expected to be politically neutral and should not make political decisions.