That's why I said "It's me" standing alone (is correct), Bert. We wouldn't, or shouldn't, launch into a sentence where 'me' is the subject, the effective nominative. of some long clause or sentence. "It's me" is accepted as an emphatic answer to "Who's there?" but "Me who is the manager of Barclays Bank who has come about your overdraft" is not ! But "Me. I am the manager..." sounds right, if a bit eccentric, because it suggests that the questioner ought to have known who it would be at their door, or so the manager thinks, and so the emphatic 'me' is used first by him, no doubt in exasperation.
English develops. All that can be done is to record eithe what educated people say, or what the vast majority say, and make that the rule for the time being.