There may not have been either a murder or a manslaughter conviction given the suicide, see s.2 (2) and (3) FA 1982. S.1 FA 1982 refers to an unlawful killing and a general public policy rule . It might be worth checking to what that term amounts.
The Forfeiture Act 1982 regulates some, but not all of the adverse effects of the general public policy rule, which is not a creation of statute. S1 is quite clear on that point, particularly when the rest of the Act is taken into consideration
To my mind, the NRB is a statutory creation, not a heritable property right as such.
Can the issue be rephrased? Perhaps asking whether the public policy rule forbids the NRB statutory provision from applying?
If put in those terms, the answer may be that s.1 and eth remainder of FA 1982 does not in fact forbid it, but seems to supports the NRB advantage flowing through when the killer in fact does not benefit from the NRB himself.
The NRB is not a property right as such.
The Forfeiture Act provides, inter alia for the Courts still to allow social security payments to dependents, and claims by dependents for maintenance etc.
You might find that the NRB is available in this case, but only because the unlawful killer does not directly benefit owing to his taking his own life .
If there is an issue s.2 FA 1982, permits a court to make a ruling on the issue, and it looks as if you might fall within the scope of that provision.
Basically, does it not boil down as to whether HMRC will challenge this? Here they might simply abstain as I would imagine that the effect upon the relatives will have been severe.