The Boeing 737 (4/500) has been rumoured to be going to BAcitiexpress for a while now, but the sticking point (one of them) is the fltightdeck crew pay. Birdseed Airways were not prepared to pay the citiexpress guys the same as mainline.
The B737 fleet may yet go to 'a subsiduary' as the summer schedule shows yet more Airbus flights T/F LGW on the domestic network, including the last one out of LGW in the evening to EDI (nightstop), and the first one out in the morning. Its listed as an A319 which is Y126 comapred to a B737-400 with Y146.