I had to edit your previous question because it asked multiple questions. Enotes does not allow you to ask more than one question, so please remember in future to not do so.
The answer to the question you asked can be found in Chapter Forty-One. This chapter begins with Lydia bemoaning the decampment of the militia to Brighton and away from the Bennets. With typical irony, Austen says that "the dejection was almost universal." However, the terrible depression into which Lydia in particular is thrown by this news is quickly cleared away when she receives an invitation to accompany her as her companion to Brighton with the militia. In spite of the misgivings of Lizzie in particular, her father agrees to this, and thus Lydia is free to go to Brighton and run off with Wickham.