Condolences for your loss.
I've brought back my Dad's ashes via QF/BA.
Quite simple. Keep it in the original container that will be sealed by the crematorium. Make sure you have the death certificate or any other formal documentation. Carry through in your hand luggage but make sure you follow the normal hand luggage rules (which change monthly). (ie, 1 piece only etc)
If the urn has been opened or the ashes are in another vessel the security guys may get a bit curious as to what the composition of the ashes are.
Once they were opened (I scattered half in Scotland, the other half in Australia) then it was easier to go into the hold as the container wasn't securely sealed. If you're thinking of doing something like that, the crematorium can split the ashes into separate sealed containers.