"And if you will stay another month complete, it will be in my power to take one of you as far as London, for I am going there early in June, for a week; and as Dawson does not object to the Barouche box, there will be very good room for one of you -- and indeed, if the weather should happen to be cool, I should not object to taking you both, as you are neither of you large.'' Snip

Right. She's bumping Dawson up to the seat with the coachman and she can take both of them if they are willing to squash in, but she can't guarantee to have a place for both if the weather doesn't cooperate. What's the other to do, ride the post alone?

BTW, Lizzie and the Gardiners are riding in a barouche. The coachman rides in the barouche box.