I'm curious if it's possible to document roughly when the split happened, and also curious whether British or American usage diverged.
If you look at this Google Ngram, you see that both Brits and Americans used to use "in future", and this expression is being replaced by "in the future" both places. It's just that in the U.S., the replacement is now almost complete while in the U.K. it is still happening.
When did this split happen? The replacement was quite gradual, so it's hard to narrow it down to something shorter than "the last half of the 19th century".