Joe, I have no idea if this will work for you, but it works for Brittany Ferries (only if you phone up or do it at the port - not online).
Try asking for a bunk in a shared cabin, they're often very cheap (BF is £15), and because so few people ask for them it's incredibly rare to share (mind you more people know now, so ....).
I've done this for years, and even better if they use those punch cards for the lock you get in quick, through your bags over all the beds and then go out taking all the cards. If anyone else comes along, they get to the locked door, go back to the reception and because they can't be bothered at reception, they just give out another cabin number, so you get a cabin to yourself.
Like I said no idea for this route, but always worth a try.
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