Im fairly certain i said this in other threads, but BA have a very small amount of A321's in the fleet. 11 frames just seems an oddly small amount to have in the fleet. with all the capacity problems between LHR & GLA it would seem an obvious choice to task the 321 with the extra seats, however it seems as if BA dont have enough A321's to use on UK Domestics as well as popular European services.
Personally i would expect to see some of BA's Airbus options taken as the A321 variant.
cs