Andrew, the problem is of course that there is no price schedule published anymore. Previously you could look at the BF brochure, works out how much it is costing, compare with CC & C&CC and take the best deal.
Not anymore you are at the mercy of BF's pricing on demand. How do we know that booking early is really a bargain. If the ferry has empty places and you book on the day of travel do you get a bargain?
I also have it on very good authority that a like for like return trip which starts in either Poole or Cherbourg have diifferent prices - guess which one is the cheapest?