If you do not qualify as next of kin (in the MOD info sheet) then you need to get permission from next of kin to apply for the full record (personal details). If not you get the basic record.
I've just applied for the record of my mum's first husband (short marriage as he died 1947); I can only get the basic as not next of kin and have been unable to find any to get permission. Think I have most of his background anyway.