Rule Britannia. Daphne Du Maurier. 1972.

Emma, who lives in Cornwall with her grandmother, a famous retired actress, wakes one morning to find that the world has apparently gone mad: no post, no telephone, no radio, a warship in the bay and American soldiers advancing across the field towards the house. The time is a few years in the future. England has withdrawn from the Common Market and, on the brink of bankruptcy, has decided that salvation lies in a union - political, military and economic - with the United States. Theoretically it is to be an equal partnership; but to some people it soon begins to look like a takeover bid.