Well , for decades, if not a hundred years, there have been limits on what weight any employee, in any business, can be expected to lift unaided. There are different limit s for men and women. Don't think that there's any special rule for baggage handlers. At Stansted Airport they divert baggage etc above a certain weight , so it goes to a different section. Presumably they have extra staff or special equipment in that section, to comply with the law as it is and long was.