>>....soldiers and sailors may simply declare their wish by word of mouth.
British soldiers who go on a tour of Northern Ireland must make out a written will.

I would have expected a written will to be very strongly recommended, if not obligatory, for all servicemen. Same applies to any high-risk job.

The nunccupative option is mostly of historical value I expect, as employers (including the armed forces) have legal obligations for the welfare of their employees these days.