OK- Continued...

[poliphilo]

steepholm has done some digging. The first appearance of "OK" in a British publication (the Boy's Own Magazine) is dated 1864. On the principle that a phrase is likely to have been current in conversation for 10 years before it first turns up in print we can assume that Brits have been saying "OK" since the 1850s.

On this first appearance the phrase is attributed to the (probably fictional) Sir William Curtis and is glossed as meaning "orl korrect". In its second appearance the following year it is explained as common usage among surveyors. This need for explanation suggests it was still relatively unfamiliar.

So, my question has been answered. It is not impossible that a Derbyshire hill farmer would have been saying it in 1916.