Runaway Bride — Jane Aiken Hodge

When tempestuous, red-haired heiress Jennifer Purchas was informed by her uncle she was to marry a stranger, she exploded. Years of pent-up resentment broke through in that moment, for her guardian uncle had made life miserable enough. And now this. To be forced to wed some odious fortune hunter. Never. Jenny fled, and took a position as companion to the formidable, old Duchess of Lewes. The Duchess suspected Jennifer was no ordinary young lady. And so did her grandson, handsome young Lord Mainwaring, who found Jenny's charms very disturbing. As for Jenny, how long could she keep her identity a secret? How long before her uncle's spies would discover her?
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UK CORONET Edition:

Jennifer Purchase stamped her foot on its Waterloo heel and glared up at her Uncle Gurning. 'I'll not have him, Uncle!'

The Duchess of Lewes was insistent that her favourite nephew George should marry at once. George himself resolved to ask for the hand of Jennifer Purchase, a suitable heiress and sister of his dead comrade-at-arms. Her uncle was delighted at his excellent offer, George prided himself on having done the right thing. Everything seemed more than amicably settled.

But no-one had thought to ask Jennifer her feelings in the matter. High-spirited and defiant, she decided that no-one was going to force that odious George Ferris on her. So she ran away. Surely she could hide hereself away quietyly as a governess until she came of age? But Jennifer was never in the least bit quiet, nor was she much of a traditional governess...