BA would have to look after them under 'duty of care' obligations.
Depends.
If it is agreed in advance by both sides that those downroute will not be considered to be "on strike" until they return to base
provided they agree to operate back to base when the company require it (ie. when an aircraft turns up), the company will continue to provide accommodation and allowances to the cabin crew downroute.
If this is not agreed, clearly the crew would not be obligated to operate back but if they chose that option, they would then be on their own ref A&A. Duty of Care would be unlikely to apply in that case as the crew would likely be "in breach of contract". Suffice to say, the lawyers would sort out any "breach of contract" after the event which wouldn't help crew at the time.