I read this maybe ten years ago but am unsure if it was new (library). Similar to SS GB.
It tells a tale of underground resistance to the Germans, in particular a policeman who plays the ruthless guy with prisoners but secretly he is part of high level resistance. The police become known as the lice because they work with the Gestapo.
A married couple get separated and, unknown to each other, form spy rings, one codenamed Carpenter (I think!) and the other codenamed Night watchman.
They work undercover with American backing. I think there is a secret funeral for the policeman at the end with Churchill in attendance because to the general populace the cop's name is scum
Edit: Originally various local village people are recruited pre-invasion. Part of Churchill werewolf activities, then the enemy swarm in. Protagonist woman is in a hidden base reporting on invading troop movements, then she has to flee for her life as German detector vans track her down, she then forms a resistance group