Is the A321 CEO line still open? They are similar capacity to the 757 (218/220) and therefore could easily replace the 757s. They could also use them to replace some 737-800's in BHX/STN and then either use the replaced 800's to continue to expand those bases OR open a new base elsewhere. Bases with A321 could quite easily be MAN, LBA, STN and BHX - not too thinly spread around the network.
Further to that in Summer when they lease Titan A321 if they replace that with a owned A321 they could send the Titan elsewhere (GLA, EDI, NCL).