From what I remember from my course in British legal history, it was not a certain official instrument that Parliament had signed that had signaled the obvious reduction of power of the British monarch, but rather, it was King Edward VII's inability to affect politics as dramatically as her mother did.

To contextualize the dramatic loss of power, the restraints of the monarchical powers had been established ever since John Lackland signed the Magna Carta and the Bill of Rights 1689 was put into law under co-monarchs William III and Mary II, but the monarch still have power to influence parliament to his (or her, as with Victoria's case) whims.

As a case point, Victoria intervened (albeit privately) in a number of laws that Parliament passed, like the Representation of the People Act 1884, in which she "encouraged" the House of Lords to pass the act to the detriment of the members of the House of Lords, as noted by Sir Phillip Magnus in his book.

Meanwhile, Edward VII, as also noted by Sir Magnus in the same work, suffered a parliamentary crisis in which the 1901/1910 "People's Budget" bill that was passed in the House of Commons was then vetoed by the House of Lords, despite Edward's private urging to Arthur Balfour, then leader of the Unionists party, and Lord Landsdowne, then leader of the Unionist Peers in the House of Lords, to pass the bill.

This event precipitated the inability of the monarch to directly influence government policy through either houses, which was further reinforced by the passing of the Parliament Act of 1911 in which the House of Lords was banned from vetoing money bills and essentially, further reduced the available means a monarch could induce or reduce the passing of the bill (through "advising" the House of Lords on what to vote for)

Sources:

• Sir Phillip Magnus, King Edward the Seventh