Checked luggage is scanned, arguably more thoroughly - technically - than hand luggage. Here is the official BA advice:
We're so sorry that you've lost a loved one and are now taking them on a journey to their final resting place.
You can take their cremated remains (ashes) onto our flights in either your hand baggage or checked baggage. We just ask that, in order to comply with regulations, you carry with you a Certificate of Death, or other country-specific equivalent. (Please be aware that this document must have been issued by a competent/official authority from your country of departure.)
We'd ask you to place the remains in a tightly-sealed and sturdy outer box or case. If you're carrying them in your hand baggage we need to advise you that full security procedures will still have to be be followed in the airport and the sealed box/urn must be x-rayed at the airport security team. Because of this, it's very important to know that if your urn/container is made of a material that prevents the x-ray screener from clearly being able to see what is inside, then the container won't be allowed through with you. (It's not policy at airport security locations to open any urn and look in it, out of respect for the person who has passed.)
Our IAG Cargo team can also help with travel arrangements for human remains, including flying your loved ones home in a coffin. The coffin doesn't need to be accompanied by a family member/friend on the same flight. More information and to get help with this is available here:
(link to IAG Cargo)
Contact IAG Cargo if you have any further questions:
>Contact IAG Cargo
If you're taking your pet's ashes with you, we need you to have the following documentation with you: confirmation of their death from a vet, along with confirmation of their cremation. The information above about packing them securely would also apply to these remains too.
Personally I would suggest taking the remains as hand luggage to avoid significant problems if they get mislaid, but equally one would hope that anyone carrying ashes as hand luggage would be checking all the time to prevent leaving them on board - I know of a situation where this caused a huge amount of trouble.