Hello
I did this for my Dad. I believe that, apart from the death certificate, they require you to be the next of kin or have the agreement of the next of kin.
And then, don't expect too much information. I received a single A4 sheet with some very basic information of ships/depots and dates. The information wasn't in a form readily understood by me, I joined a navy forum to decipher it. For instance it said MMU 19 and dates. It turns out that MMU stands for Mobile Maintenance Unit - how would you know that?
Regards
Suds