Just, already and yet

I've always used the present perfect tense with "just", "already" and "yet", as in "I've just arrived...he's already finished....we haven't eaten yet". What's more, when teaching English as a Foreign Language, I noticed that all textbooks for learners of British English not only encouraged but demanded the use of the present perfect in such expressions.

However, over the past few years, I've encountered more and more Brits using the simple past, as in "I just arrived" or "he already finished", etc, etc. For me, this is American usage. In my book, "I just had lunch" means that I only had lunch  (i.e. I did nothing else). It's got nothing to do with communicating that I've recently finished my meal.

I suppose this is another example of generational change, of how language evolves and leaves older (gulp!) users behind. Even so, I still can't bring myself to use the simple past with "just", "already" and "yet". Does this mean I'm becoming anachronistic myself? What tense do you choose in this type of context?