What I was trying to say was that a diocese and an autonomous jurisdiction are practically the same - all dioceses are autonomous (in theory).
But I didn't know that a Metropolitan had the extra power to call a local synod, thanks for the info. (btw doesn't that make a Metropolitan identical to a Patriarch - with the only difference being honorific?)
I don't think they should be the same thing... If one Coptic diocese had a different Liturgy than the rest that would be weird, at least... But the BOC uses a different Liturgy... a greater degree of independence.
It doesn't make a Metropolitan the same as a Patriarch...
For us and the Greeks, Patriarch >= Archbishop > Metropolitan.
For Russians, the meaning of the title Archbishop and Metropolitan are flipped.
So if you look at how the Antiochians works since they use the same naming conventions as us, they have a Patriarch in Antioch, and they have a Metropolitan in North America. Metropolitan Philip is president of the synod of about a dozen bishops the Antiochians have in North America, but is not equal to the Patriarch of Antioch, but is a member of the Synod of bishops of Antioch, under the Patriarch.
If you look at the OCA, they have a greater degree of autonomy from Moscow. They were made independent, so their primate is called a Metropolitan (keeping in mind that Russians switch it, so that's equivalent to an Archbishop to us). So the Metropolitan of the OCA is not a part of the Synod of the Moscow Church, they are the president of the Synod of North America, and the same as the Moscow Patriarch in that respect, though less in honour since a part of a smaller and younger church.
In most points of history the Coptic Orthodox Church only existed in Egypt, and consisted of the Archbishop of Alexandria, (whose historic titles include "Pope" and "Patriarch"), and the other bishops of Egypt, making up a Synod of the land of Egypt... but their number was in the order of a dozen, not 120.
Now we have Metropolitans as well. I'd be interested to know how the title was historically used in Egypt, but I have no idea. Imagine that we used it in a meaningful way that was consistent with how we've used it with the BOC and French though...
Say the Pope was actually Archbishop of Alexandria in reality (i.e. lived there and actively worked as the father of that Church). Now, you wouldn't make a bishop of Alexandria, and say "hey, you've been doing a good job for a while, let's make you Archbishop and give you the title Pope now"... as soon as they are ordained the bishop of Alexandria, they also receive the additional duties that go along with that archdiocese, namely being the Archbishop, or Pope and Patriarch of Egypt.
Now say the Pope wasn't actually the bishop of Cairo, then certainly the bishop of Cairo would be a Metropolitan bishop, there is no arguing that Cairo isn't a great city. They wouldn't be made a bishop, then later promoted to Metropolitan to recognize seniority in the Synod, they would be a Metropolitan bishop because they were the bishop of a Metropolitan city, just like the bishop of Alexandria is an Archbishop because they are bishop of an Archdiocese. Of course, there would be other bishops around Cairo as now, a bishop for Old Cairo, for Shoubra and for Shoubra el Kheima, etc. The Metropolitan of these bishops would be the Metropolitan of Cairo. The Archbishop of these bishops and their Metropolitan would be the Pope of Alexandria. In pastoral and local matters affecting Cairo, the bishops of Cairo and surrounding areas would meet as a local Synod. When discussing issues affecting the land, the bishops would meet in a great Synod under the Pope. When discussing issues affecting the whole world, they would meet in an Ecumenical Council, presided over in turn by the prominent Patriarchs, as happened at Nicea, Constantinople, and Emphasis...
Egypt has never had this structure. It has always been just the Archbishop of Alexandria and the rest... but that makes less and less sense as the Synod gets larger and larger, and covers the whole world.
So the French and the British were each given their own Metropolitan. The French had another bishop in their local Synod, the British have not so far. They were allowed great freedom in deciding their own matters, such as having their own Liturgy. If France was previously large enough to be a Metropolitan area with independence, then it doesn't make sense for their second primate not to have the same title and honour as their first did.
of course, the local Coptic bishops should be part of the Synod of France or Britain, under the Metropolitan there, not separate, parallel, and overlapping jurisdictions... but that's another issue.