The Cymry Ring — Michael Allen Dymmoch

A “skillfully written, wonderfully entertaining, and fascinatingly detailed” time-travel adventure from the author of the Caleb and Thinnes mysteries (Booklist).
 
British detective Ian Carreg never expected his life to unfold like this. A recent widower, the fifty-five-year-old inspector has just learned he's going to be a grandfather. But duty still calls. He's been assigned to find and arrest Dr. Jemma Henderson, the daughter of a famous British physicist, for extradition to the United States, where she's been convicted of murdering her lover.
 
But when he pursues her to the ancient stone monument of Cymry Henge, he is knocked unconscious and awakens in what appears to be Roman Britain in the year sixty AD. Ian is convinced he's in the middle of some elaborate hoax -- until he comes face to face with Celts and Romans and begins to doubt his sanity. When he and Jemma are taken prisoner by the Romans and forced to travel to Londinium, Ian realizes they must work together to foil a plot that could radically alter history . . .