I think the plan was for the ex-BD frames to have left the fleet some time ago, but a few things happened
1) The collapse of Monarch and IAG’s purchase of their slots at Gatwick.
2) Delays to deliveries of the 787 and 777X at LHR.
3) COVID and the retirement of the 747s.
4) Order of the 737MAX for LGW - this seems to have changed and all the 737s will be for Vueling.

BA never seem to have been very keen on the A321 at LHR, is there something about noise/landing fees that that makes it unattractive at LHR? If that remains the case I could see all the A321CEO’s migrate to LGW to join the 10 G-GATx as more A320N and A321N arrive for LHR and the A319s shuffle away.