The Lady’s Companion by Carla Kelly

Daughter of a baronet who has gambled away everything, lovely Susan Hampton has no choice but do the unthinkable: get a job. Swallowing her pride, she finds a position as a lady’s companion in the Cotswolds to Lady Bushnell, a widow whose husband served with distinction in the British Army during the Peninsular wars in Spain.
Proud Lady Bushnell doesn’t want a lady’s companion foisted on her by caring relatives. Neither does the bailiff on one of the Bushnell properties. David Wiggins is a former sergeant in Lord Bushnell’s Fighting Fifth Infantry. A Welshman and product of a workhouse, David is clever, honest, and capable. He sees no place for someone like Susan Hampton. Until he does.

A lady’s companion is hardly a more improbable job for someone of Susan’s background, but she is out of choices. And there is hardly a woman less suited for David’s life. How in the world did they fall in love? How can this possibly end?