Compensation is not by weight. They can purchase any essential clothes/etc that are in the case and claim it back from BA. You won't receive compensation if the bag is found and delivered though.
Ah yes, I see now the rules on this changed some time back, been a while since I had actual loss/damage rather than delay.
They have all the essentials they need in the checked bags, what was in the carry on was new, more valuable items that my mother chose not to check in as didn't want them lost. At least the receipts will be easily available.