Unless he was more specific, like: "My estate, Combe Magna, is near the town of X", I don't think that Eliza could find it easily. Was there some kind of list of estates in England that somebody like Eliza could consult (if the idea could occur to her)? I can't imagine her traveling through Somerset looking for Combe Magna; at least, that would require money, which she lacks.

It's more likely that Eliza looked for him in London.