I work at LHR and many people travel through with ashes.
I can't comment on any particular airline's policy ( but to be honest most airlines would have no idea how many pax carry ashes in handluggage with them)
From an airport point of view, a few tips.
Make sure they are in a container that seals tightly and won't open if dropped. But the container will need to be openable should your bag get pulled for bagsearch.
Carry the container near the top of your bag.
All the best with your journey.