known for years in advance that war was imminent and the British government had committed itself to join,
I have seen no evidence for that.
Do you have any?

Known in advance: in France and in Germany the newspapers were full of it (by 1910), so at least British intelligence must have heard about it. Imminent does not mean unavoidable, but probable enough that any government failing to prepare for it would have been guilty of negligence and stupidity. However, too ostentatious a mobilization would have destroyed the propaganda story. (Again it must be stressed that many Germans and Austrians believed as honestly as Britons did that war was being forced on them by the enemy, respectively. Without that propaganda, recruiting would have been much more difficult. Only the tsar thought he could do without much propaganda because of his autocratic power - an error, as we know.)

Committed itself to join: by the treaties, as you (Keith) never fail to mention.