The Issue is Duty of Care
I suspect there may well be a Health and Safety prosecution brought against BA prior to any civil action by passengers for compensation.
The overarching legal question is whether the airline did everything it could have done - or should have done (i.e. according to its own operating procedures and crew-training)_ to ensure the safety of its passengers - and whether those procedures were adequate.
It seems already well-established that the passengers were not alerted to adopt the brace position - and this may be held to indicate a major omission in an operation where passenger safety is paramount. The fact that the passengers were then evacuated swiftly is to the credit of the cabin crew - but it was damn lucky for all concerned that there was no fire.