Just to let you know the flight was booked on BA.com and it was over the Jubilee weekend last year. Interestingly my outbound MAN to LGW flight was cancelled and when I was rebooked it did book into Y, the return however stayed in Q.
Seems strange that a domestic connection to CE can result into being booked into Q thereby denying lounge access whereas a domestic connection to WT Plus will book into Y/J and give lounge access on the domestic.