The last time a British monarch actually vetoed a bill passed by Parliament was Queen Anne's denial of the so-called royal assent to the Scottish Militia Act of 1708. King George V did come close to vetoing a parliamentary bill in 1914 granting local self-government to Ireland, though.
Elizabeth II can create an epochal moment in history by becoming the first ruler of the United Kingdom since Anne to refuse the royal assent to a bill passed by Parliament.
The parliamentary bill in question can be a very minor piece of legislation (i.e. stating that blue is the national color of the UK).