To Conquer the Throne — Tim Sullivan

The alien Visitors, under their sadistic commander, Medea, plot to seize Great Britain as a launching pad for the conquest of Earth. Parliament has been infiltrated, and the most hallowed shrines of democracy are about to be desecrated. Gabriela Nicks, an American living in England, forced to join a Resistance that is fragmented and possibly infiltrated by the enemy, races to rally the fighters of Earth for one desperate strike that may save the world, and all hope rides with a small force facing insurmountable odds and sure defeat.