Funny, I have a friend that works for a scandinavian airline (not the british special forces) that I recently asked just this (though without mentioning the FD element).

For them at least, cancelling a seat well in advance, giving the airline the opportunity to fill it + keep your money is a good deal for them that doesn't make it into market analysts' reports. Empty but paid seats however regularly show up in statistics and garner attention because it means they could have accommodated another pax.

This of course all assumes you can cancel the ticket by dial tone calling or online. You still don't want to talk to an agent.