Bigpants,
There are various sets of terms and conditions because at the moment BA can and do pay different rates for the same job across the whole of BA and are doing nothing wrong in this.
Look at CFE post 1999 when it was wholly owned by BA, their pilots were doing the same job as a 737-500 driver at BA EOG yet the pay was different.
Look at all the people who work at LGW, they do the same job as their counterparts at LHR but for less money and worse T&C.
Look at the Cabin Crew pre and post 1997 yep same job different pay.
For the time being CX CC would be pi$$ing money against the wall consulting with a barrister on this topic.