I think that JA's dislike of London is more to do with morals than discomfort. And in a lot of English fiction there is a tendency to equate London with immorality and decadence, and the country with decent purer living. William Cobbett the writer referred to London as the "Great Wen" and Mrs Austen referred to it as a place where one "didn't have time to do one's duty to God or one's neighbour". The squirearchy might visit London occasionally for getting a daugther married or business but they preferred their country homes.