Marriage Licenses
According to the Inheritance Act, the first 6 people in line to the throne aged up to 25 years have to obtain a marriage license from the Queen. If she says no, then there is no wedding.
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Marriage Doesn’t Equal Throne
The Queen’s husband, Prince Philip, is the King-Consort, which doesn’t give him the right to become King. He is native to Greece and thus has no claim to the throne. The same is true for with Prince William. If he becomes King, then Catherine will have the title of Queen-Consort.