As a previous Editor, 2007 to 2017, I can confirm that all unsold copies in newstrade – principally WH Smith, are pulped once off-sale.
It is not financially worthwhile to pay WH Smith to return them, particularly nearing in mind the huge but now diminishing, range of magazines they normally sell.
As you have gathered, WH Smith doing do anything without being paid!
Normally on a print run, there will be those for newstrade sale (across the world such as WH Smith) plus those for all the currently valid subscriptions with a few spares for those subsequently lost in the post + a few for back issues but the days are long gone of having a room in head office, such as it is, filled with back issues.
In practice, many magazines are often passed-on within a club, so my own club now has the mower shed having a shelf of unwanted but useful, magazines on it.
Paul.