For info, I did a quick comparison just now for a Plymouth-Roscoff September crossing and the BF and CMC prices are identical, at £735, however the F&F code price is £100 more than CMC if you add 4 site vouchers to your crossing.
I totally understand the quest for discounts, especially when booking something like a ferry trip to Roscoff. While I don't have an F&F code to share, I've got a hot tip that could save you some serious dough.
Recently, I stumbled upon https://megakupony.cz, a nifty website that's a goldmine for finding real coupons, not just for travel but also for online stores. Trust me, they've got some juicy deals waiting to be snatched up.
As someone who loves to save a buck or two, I've personally used this site and found it super helpful.