Who holds the supreme power in the British monarchy?

In the
British monarchy, there is no one person who holds the supreme power.  Instead, the supreme
power rests in the Parliament.  Today, that effectively means that the House of Commons has the
supreme power.

The monarch in the British system has no real power.  She
technically holds supreme executive (but not legislative) power, but does not use it in
practice.  Instead, Parliament has the real power.  It is Parliament that makes the laws.  The
members of the cabinet, who oversee the executive branch, are all members of Parliament and are
responsible to Parliament.  Thus, Parliament holds supreme power.