The 2 local nights rest post long range which is, at the moment, the cause of such disgust during this disruption, forcing BA to strand passengers and operate empty aircraft, is not a legal restriction put in place by the CAA, it is a union agreement enforced inappropriately by BASSA. As has already been mentioned the legal restrictions governing Flight Crew are ALWAYS more restrictive. Hiding behind restrictions on duty hours imposed by an industrial agreement, during times of disruption is a red herring - if the pilots can LEGALLY do it so can the cabin crew. The rest is just about how little you care about pissing off passengers.
You can operate with less than 2 nights post long range - the 122MRU service that diverted into NCE did just that. The boys and girls bussing back from PIK because they insisted on their 2 nights did not. Ultimately the Union should be about protecting you from the excesses of the company - not dictating to the company how and when it can fly its aeroplanes.
It is not any reduction in cost of cabin crew that will be the main benefit that BA will reap from OP Colombus, but flexibility from one of its most industrially intransigent groups of staff. BA is losing about £2.7 million a day and do not need to incur additional losses and costs because any group of staff is overly inflexible when things go wrong. The more inflexible and awkward you are at a time like this will make BA even more determined to push ahead with reform.
Get a grip guys you will regret it in the long term if you don't. When BAs cash reserves are gone there will be no more old contract or new contract or box payments etc. Anything that emerges from the wreckage will pay less and work all its crew to CAA rules and nobody wants that.