I don't think they'll be marooned. I believe BASSA / UNITE have advised cabin crew to complete their scheduled trips before commencing strike action.
Pretty hard to complete scheduled trips back if the aircraft they were due to operate on has been stranded in LHR due to striking crew...
e.g. if a crew operate into JFK on the late afternoon (local) of the 21st and the aircraft then turn around with new crew and head back to LHR, arriving on the morning of the 22nd how are they going to operate a flight back on the evening (JFK time) of the 22nd if no aircraft have left London (IIRC there is only 1 BA nightstopping at JFK)? Or will they be refusing to operate on the 21st too?