The Gypsy and the Gentleman by Beverley Oakley

Impersonating the cousin of a debutante who is to attend Lady Quamby’s “Instruction in the Art of Graceful Accomplishments”, Tilly finds herself out of her depth when she falls for the man she believes is just the lowly dancing master.
But when their roles are reversed and Tilly discovers Henry is about to inherit an earldom, can she accept his marriage offer in good conscience?
For while Henry is determined that the disparity in their social standing will not prevent their happily-ever-after, Tilly knows something – or someone – who will.

The Gypsy and the Gentleman is Book 8 in the Scandalous Miss Brightwell series but can be read as a standalone.