no hand baggage weight limit on BA as long as you or someone with you can struggle it into the overhead there will be no problem. The overhead lockers are easily up to heavy bags. The Cabin crew are not supposed to assist you (if they did they would risk injury and no one should have to do that at work) but sometimes fellow passengers will assist, I find college age American men to be very polite in offering to help. I always help mothers with babies and elderly ladies if needed