Perhaps some of our UK friends can help. I'm not familiar with the concept of a travel co-op.
I do know that Carnival, part of the same group, offers an Early a Saver fare that acts like what I think you are describing. You have to book this fare specifically in order to benefit from future price drops.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014
Star Clippers: Royal Clipper 2015