Ref the LHR-GIB change - Extra slots came available and as there is a good cargo and pax business ex LHR-GIB, GB now do LHR-GIB-LGW-GIB-LHR aircraft rotation.
Simply, you will fly on GB which operates as BA, (argueably to a better standard on a lower cost base.) So you will see very little difference apart from a GB sticker on the nose of the A320.