If Air China were granted 3 flights a week Beijing - Manchester, they are only going to agree to BA doing the same , if the reciprocity was from LHR , they are not going to agree !
I do actually have some sympathy with BAs stance, why would you let somebody into your backyard but how on earth we square this I really do not know and how the MEBs were allowed in again no idea !
Easily reciprocity can be restricted to equal value routes and does happen in some bi-latterals
Example CAAC Air China (Or other branch) awarded a regional PEK-MAN license . BA (VS other UK carrier) receive a LHR- Regional City Chinese license (Not more PEK/PVG) or a opportunity to compete directly MAN-PEK.