Marine Insurance Act 1909

Where, after the commencement of the risk, the destination of the ship is voluntarily changed from the destination contemplated by the policy, there is said to be a change of voyage.

Unless the policy otherwise provides, where there is a change of voyage, the insurer is discharged from liability as from the time of change, that is to say, as from the time when the determination to change it is manifested; and it is immaterial that the ship may not in fact have left the course of voyage contemplated by the policy when the loss occurs.