It's a fairly simple question, I think, that is causing some very confusing answers;
I think the reason for that is because of the way you asked the question.
You didn't state the pilot in question had an FAA certificate, nor what privlidges were on the certificate. It read like you were considering the JAA MEP licence as the "foreign licence" under ICAO, and trying to get some sort of implied privlidges.
Given that you've now clarified that, you should get pretty consistant answers, inline with what Bose just said. But be aware that that is correct for the UK, but not necessarily correct for other JAR countries. It's a UK rules, rather than a JAR one.
dp