Which spy writer imagined a Nazi-occupied Britain in his novel 'SS-GB'?

SS-GB is an alternative history novel by Len Deighton, set in a United Kingdom conquered and occupied by Germany during the Second World War. The novel's title refers to the branch of the Nazi SS that controls Britain. It was first published in 1978.

SS-GB is set less than a year after Britain’s surrender following a successful Operation Sea Lion. In 1940, the Germans landed near Ashford, Kent, and Canterbury was declared an open city. The German advance captured London but a British rear guard around Colchester slowed down the Germans for long enough to enable Royal Navy ships to escape from Harwich.

The King is a hostage in the tower, the Queen and Princesses have fled to Australia, Churchill has been executed by a firing squad, Englishmen are being deported to work in German factories and the dreaded SS is in charge of Scotland Yard. London is in shock. The very look of daily life is a walking nightmare of German uniforms, artifacts, regulations.