[From P Schofield Apr 08]

This is just a guess - while the armistice was signed in November 1918, Britain and Germany were still formally at war until the Treaty of Versailles in June 1919, which may be why  those behind the Hyde Park memorial chose that year to mark the war's end.

[From B Hogg Jun 08]

although fighting
stopped on 11th November 1918, that was just the armistice. The
negotiations went on for months after the ceasefire and the Treaty of
Versailles was only signed at the end of June 1919, marking the
official end of the War.

http://www.worldwar-1.net/ seems to be a good resource for this sort
of thing.

[From J Horsman Jul 08]

The war officially ended in 1919. The fighting stopped on November 11th, 1918 but all nations concerned were in a 'state of war' until the 28th of June 1919, when the Treaty Of Versailles was signed. On 21st June 1919, 9 German sailors were shot dead; these could be considered the last casualties of the war.