From the Imperial War Museum web site ...
Who might have received the plaque, scroll and King’s message?
The immediate next of kin of all who died serving with the British and Empire forces in the First World War were eligible to receive the plaque and scroll. With nearly a million dead for the British Army alone, the plaques are today still commonly found; the fragile scrolls survive less often. Some of those recorded by plaques and scrolls were not eligible for service medals, for instance, those who did not serve overseas but who died in service through accident or illness. Deaths in the period 1919–1921 (possibly later) could still lead to the presentation of a plaque, if either still in service (and even from natural causes), or the death was accepted as war-related. It is thought that many British and Empire war dead had no plaques and scrolls issued, due to the inability by 1919–20 to trace addresses for the eligible next of kin – a result of the high incidence of short-term rented addresses, re-marriage, and that, if they died unmarried with parents dead, there might be no dependants claiming a pension.