The two stations have common routeing points so there are no mapped routes in the Routeing Guide. This means you must use a direct train or a route no more than 3 miles longer than the shortest route, unless there are any relevant easements (which there are not).
The only such route for a ticket marked ANY PERMITTED is via Bolton.
Going via Preston is not valid, unless you buy the more expensive ticket, marked VIA PRESTON. Confusingly the VIA PRESTON ticket would also be valid by Bolton, as the Any Permitted is cheaper.