The story of the unknown warrior : 11 November 1920 / Michael Gavaghan. (OCLC #908110563)
The LC class number D680.A-Z is for WWI celebrations, memorials, and monuments in regions or countries other than the United States (with the cutter starting with A-Z specifying that region or country).
This cutter is taken from a predefined list, the Regions and Countries table from the Subject Cataloging Manual on Shelflisting (G 300).
The cutter for Great Britain is .G7, so this book gets the call number:
D680.G7 G39 2003