The same pax 'scam' applied to the channel crossings too. Get two cheap day-returns. Use one out and later use the other back. Easy to say you missed the first return ferry. Difficult to argue that you had 'missed' the second outbound, ie first sector.
Cross channel ferry companies are getting wise to all sorts of loop holes, especially trying to buy a day return ticket for a ferry leaving after lunchtime (for the outward leg) as that costs far less than a single.
Single tickets be they for flights or ferry crossings are EXTORTIONATE and certainly more than twice the cost of a return ticket ...
Oh well, the world is not a fair (fare?) place is it ..