The Cruel Stars. Ark Royal, #11

A stand-alone set during the Ark Royal Era. The Royal Navy never expected to fight a full-scale interstellar war.  Everyone knew the Great Powers would never risk everything on armed conflict, when there was plenty of room for everyone in outer space.  But when a hostile alien force stumbles across humanity's handful of colony worlds, the Great Powers must set aside their differences and fight to preserve humanity from utter destruction.
Desperate for starships and manpower, the Royal Navy embarks upon an ambitious plan of converting freighters into makeshift carriers and recruiting reservists and criminals to fill the ranks.  Classed as expendable, the small carriers will be given the most dangerous missions to slow a remorseless alien foe ...... And the pardons their crews have been offered will be meaningless if they die.