I'd like to ask about your use of "just" first.
When you use "just" to refer to time, it means it happend moments ago.
Despite multiple claims that Americans don't use the present perfect, I assure you that we do -- however, I have found that the use of "just" is one example when we do seem to use it differently than our other English-speaking friends.
I would say "I just saw him." [Note where I have put "just."]
Based on other threads here, I believe British speakers would say "I have just seen him."
Your position of "just" is different. He is the only person you saw - you saw "just him, and him alone."
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