The reason the unions didn't object to the LGW contract, as it is now is because it had actually been in existance since 1991 when BA bought Dan Air. SF LGW is basically the Dan Air contract with 777 flying added to it.
Objections weren't raised because it meant that the existing WW LGW crew would be sent to LHR (something many wanted) and EF LGW would get to do long haul flying.
This new thing at LHR is different entirely. A lot of people want to do 'mid-fleet' again but not on inferior conditions.
With regard to inflexibility, it cuts both ways. I've lost trips (and flight pay to boot) for the sake of 10-20 mins. Perhaps that is something they can look at?