Should Van Rompuy claim Primae Noctis
In fact as, European Union law takes precedence over UK law the government wouldn't need the agreement of 15 Commonwealth nations.
The Government could use the European Communities Act 1972 to override the Statute of Westminster 1931.
Just hope that the happy couple are both fertile. Can you imagine the kerfuffle if they announce the conception of the first 'Test Tube Heir to the Throne'