I suppose I first became properly aware of ejectives being used in English about twenty years ago, when I noticed a couple of my colleagues at work (non-linguists both) using them. I don’t know of any systematic study of their use, although a poster paper was given at BAAP in 2006, here, dealing with their use among Scottish pre-school-age children.

Daniel Jones (An Outline of English Phonetics, 9th edition, reprinted 1969) mentions ejectives only because, he says, French speakers sometimes use them when speaking English. Gimson (An Introduction to the Pronunciation of English, 2nd edition, 1970) says they occur in “Northern types of British English” (p.34). John Wells’ Accents of English, although I have not re-read it to confirm this, seems not to deal with ejectives at all - they do not figure in the index to any of the three volumes.

To my ears, ejectives, particularly [k'] are occurring with ever increasing frequency.

My impression is that they must have arisen some time ago, whenever it was that the glottal stop first started to replace the alveolar plosive. My assumption is that the progress of the sound change is as follows:

glottal reinforcement > glottal stop > ejective. The ejective arises in order to distinguish more clearly between the various plosive phonemes. It occurs mainly at the end of phrases, usually, but not always, to add emphasis to a stressed syllable.

The earliest example that I’ve heard is in the original film of Chitty Chitty Bang Bang, and was used by Lionel Jeffries (1968). There are probably examples in earlier British films of the 1950s but more likely 1960s, such as The Wrong Arm of the Law, when non-RP accents started to appear spoken by genuine non-RP-speaking actors, rather than non-RP parts acted by RP-speaking actors.