I took my mother's ashes on FlyBe last year (BHX-IOM). They had info on their web site.
Basically, there are two rules that all carriers ask:
- Have the correct certificate from the Crematorium.
- Ashes to be in a plastic container so that it can be x-rayed.
You could just as easily put them in checked luggage. I should add, I work in a related field and knew the rules before checking with FlyBe.