I have been wondering why the Gardiners invite Lizzy to tour the Lake District with them when she passes through London on her way back from Rosings but then change the invitation to the Peak District (which I kept hearing as the "pig district" in P&P3--LOL, I must have had "that pig" on my mind or Mr. Gardiner's girth--and sadly lacking in UK geography).
The only reason I can suppose is (from the author's point of view) to have Lizzy tell Wickham at the "Off to Brighton" farewell party that she will be traveling but not to let him know she would be touring Derbyshire, which might change their conversation.
Otherwise the Gardiners' invitation could have from the start been to tour Derbyshire, unless JA thought that, so soon after the first proposal, Lizzy would have found an excuse not to accept an invitation to travel so near Pemberley; once plans changed in June, she was already committed.