British Airways’ policy on hand luggage is any weight is fine, as long as you can lift it into the overhead bins without assistance.
The heaviest Atlas Stone lift is 560lbs, which seems like a reasonable answer to your question.
It looks like Atlas Stones are generally mold-poured concrete, so you'd have to pour concrete into a rectangular shape.
What is the maximum mass (in pounds) of concrete that can be poured into a carryon luggage rectangle compliant with BA policy?
EDIT: 51 pounds, because that's the BA published limit on carryons. So, 16"x12"x3.451" using 133 lbs/ft^2 pre-mixed concrete, best case scenario. Make sure to take the handle's weight into account.
https://www.britishairways.com/en-us/information/baggage-ess...
https://www.calculator.net/concrete-calculator.html?slableng...