Their Darkest Hour — Christopher G. Nuttall

...In Britain's darkest hour, a hero will arise... When alien starships from a hostile interstellar power arrive in orbit, Britain is one of their first targets. Swiftly, the aliens take control of Britain's cities and force the remainder of the British military to go on the run. With the government destroyed, the population must choose between fighting and collaborating with the alien overlords. But resistance seems futile and the aliens appear unstoppable â€" and the entire population is caught in the middle. As the alien grip tightens, the last best hope for freedom lies with those who will never surrender...and are prepared to pay any price for the liberation of Earth.