Match the window before the distance.
Pick one coastal centre for each proposed window and keep both local pages visible rather than merging the cities into one pool.
Starting around Southampton
At the Southampton end, use “Two coastal cities should remain separate discovery markets despite regular travel between them.” as a scheduling prompt rather than a claim that the whole route is local.
Planning around Portsmouth
At the Portsmouth end, pick one coastal centre for each proposed window and keep both local pages visible rather than merging the cities into one pool.
Compare both local pages
Treat each town as a separate local context, then use the overlap planner rather than assuming the full route is convenient at every time of day.